Product Documentation

Créer

Jan 25, 2017

Overview

Designing a desktop virtualization solution is simply a matter of following a proven process and aligning technical decisions with organizational and user requirements. Without the standardized and proven process, architects tend to randomly jump from topic to topic, which leads to confusion and mistakes. The recommended approach focuses on working through five distinct layers:

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The top three layers are designed for each user group independently, which were identified during the user segmentation section of the assess phase. These layers define the users’ resources and how users access their resources. Upon completion of these three layers, the foundational layers (control and hardware) are designed for the entire solution.

This process guides the design thinking in that decisions made higher up impact lower level design decisions.

Layer 1: The User Layer

The top layer of the design methodology is the user layer, which is defined for each unique user group.

The user layer appropriately sets the overall direction for each user group’s environment. This layer incorporates the assessment criteria for business priorities and user group requirements in order to define effective strategies for endpoints and Citrix Receiver. These design decisions impact the flexibility and functionality for each user group.

Endpoint Selection

There are a variety of endpoints devices available, all with differing capabilities, including:

  • Tablet based
  • Laptop
  • Desktop PC
  • Thin client
  • Smartphone

The user’s primary endpoint device must align with the overall business objectives as well as each user’s role and associated requirements. In many circumstances, multiple endpoints may be suitable, each offering differing capabilities.

Decision: Endpoint Ownership

In many organizations, endpoint devices are corporate owned and managed. However, more and more organizations are now introducing bring your own device (BYOD) programs to improve employee satisfaction, reduce costs and to simplify device management. Even if BYOD is a business priority, it does not mean that every user should be allowed to use a personal device in the corporate environment.

Certain user requirements, which were identified during the user segmentation, can greatly impact the suitability of personal devices:

  • Security – Users requiring a high-level of security might not be able to bring a personal device into the secured environment for risk of data theft.
  • Mobility – Users operating in a disconnected mode might not be able to use a personal device, as the local VM desktop VDI model associated with this type of requirement can have specific hardware requirements, or special maintenance requirements.
  • Desktop loss criticality – Users with a high desktop loss criticality rating might require redundant endpoints in the event of failure. This would require the user to have an alternative means for connecting in the event their personal device fails, likely making these users poor candidates for a BYOD program.
  • VDI models – A personal device should not be recommended for user groups utilizing a local VDI model like a local streamed desktop, local VM desktop or Remote PC Access. These VDI models typically require a specific hardware configuration or installation that will restrict device selection.

The following diagram provides guidance on when user owned devices could be used:

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Decision: Endpoint Lifecycle

Organizations may choose to repurpose devices in order to extend refresh cycles or to provide overflow capacity for contract workers. Endpoints now offer more capabilities allowing them to have longer useful lifespans. In many cases, these hardware capabilities vastly outstrip the needs of a typical user. When coupled with the ability to virtualize application and desktop workloads, this provides new options to administrators such as repurposing existing workstations. These options go well beyond the simple three-year PC refresh cycle. However, the benefits of repurposing or reallocating a workstation should be balanced against the following considerations.

  • Minimum standards - While cost factors of repurposing existing workstations may be compelling, certain minimum standards should be met to guarantee a good user experience. At a minimum, it is recommended that repurposed workstations have a 1GHz processor, 1GB of RAM, 16GB of free disk space and a GPU that is capable of supporting HDX features.
  • Business drivers - Priorities underpin the success of any major project. Those organizations that have prioritized reducing capital expenditure by means of prolonging the hardware refresh cycle can benefit from repurposing hardware. Conversely, if an organization’s business drivers include reducing power consumption as part of an overall green initiative, purchasing newer endpoints may be beneficial in order to take advantage of the latest generation of power management capabilities available in the most modern devices.
  • Workload - The type of work and VDI model for an end user can determine whether they are a good candidate for a repurposed endpoint, or may be better served with a new device. If the work performed by the individual involves locally installed applications, the individual may be best served by a new endpoint that offers the most powerful and recently updated processor and graphics architecture. However, if a user is largely performing tasks associated with virtualized applications that do not involve the latest multimedia capabilities such as webcams, VoIP and media redirection, then a repurposed workstation should be a viable alternative.

The following planning matrix outlines considerations when repurposing existing hardware:

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Decision: Endpoint Form Factor

The capabilities of endpoints have grown along with efficiencies offered in thin client form factors. Even mid-range thin clients now have graphics capabilities that allow utilization of HDX features such as multi-monitor support while offering management and power efficiency benefits. Citrix has developed a three-tiered classification for thin clients based on their HDX capabilities: HDX Ready, HDX Premium, and HDX 3D Pro, which can be used to help narrow the field of appropriate thin client devices based on the use case requirements. This expansion of capabilities has given IT administrators more options and flexibility than ever before.

Most organizations will likely deploy a mixture of fully featured clients as well as thin clients. However, certain endpoint devices are more appropriate when used in combination with certain VDI models as explained in the following table:

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Receiver Selection

Citrix Receiver is an easy-to-install software client that provides access to applications, desktops and data easily and securely from any device, including smartphones, tablets, PCs and Macs.

The following section provides a series of design decisions that should be considered when deploying Citrix Receiver.

Decision: Receiver Type

While most organizations should simply deploy the latest Citrix Receiver compatible with their endpoint, it is important to recognize that there are certain differences between editions. The following table should be referenced to determine the most appropriate edition of Citrix Receiver for each user group. For the latest feature matrix, please refer to Receiver Feature Matrix.

Decision: Initial Deployment

There are several deployment options available for delivering Citrix Receiver to an endpoint. Although it is usually a best practice to have a full version of Citrix Receiver deployed to an endpoint to provide the greatest level of functionality, it is important to consider fallback options such as the HTML5 Receiver for those situations where the installation of Citrix Receiver is simply not possible. Note that although the HTML5 Receiver can be used as a fallback option, like the Java client was with Web Interface, it is not generally recommended as the primary Receiver for enterprises to standardize on due to the limited feature set and common browser restrictions around unsecured WebSockets connections (see CTX134123 for more information).

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The following mechanisms are commonly used to deploy and update Citrix Receiver:

  • StoreFront - If Citrix StoreFront is available, administrators can deploy Citrix Receiver via a Receiver for Web site by enabling the “Client Detection” feature. When deployed, a Receiver for Web site enables users to access StoreFront stores through a web page. If the Receiver for Web site detects that a user does not have a compatible version of Citrix Receiver, the user is prompted to download and install Citrix Receiver. The Receiver clients can be hosted on the StoreFront server, or users can be directed to citrix.com for the latest Receiver files.
  • Internal download site - Users may be prevented from downloading software from the Internet, even if they have permission to install applications. Administrator can create an internal website for supported Citrix Receivers or host them on a common software distribution point for a more seamless user experience. This could be an alternative to enabling Client Detection on the StoreFront Receiver for Web site, which can result in an inconsistent user experience depending on browser’s ActiveX settings.
  • Markets and stores - Citrix Receiver is available on the Windows, Android and iOS stores..
  • Enterprise software deployment - Many organizations employ an enterprise software deployment (ESD) or Mobile Application Management (MAM) solution. ESD/MAM solutions can be used to deploy Citrix Receiver to managed endpoint devices. Employee-owned devices can only be managed if the user successfully registered the device with the management tool.
  • Master image - Most organizations have a group of master desktop images, which are deployed to each business owned desktop, laptop, server, or virtual desktop. A common mechanism to ensure access to virtual desktops and applications is to include a supported version of Citrix Receiver in the master image. Subsequent updates to Citrix Receiver are handled either by enterprise software deployment tools or manually.
  • Group policy - For customers without a robust ESD solution, it is possible to deploy and configure Citrix Receiver via Microsoft Group Policy. Sample start-up scripts that deploy and remove Citrix Receiver are available on Citrix XenApp and XenDesktop media:

Citrix Receiver and Plugins\Windows\Receiver\Startup_Logon_Scripts

  • Manual install - All supported versions of Citrix Receiver are available from the Citrix Receiver Download site. Upon landing on this site, client detection is performed and a platform and operating system specific link is provided to allow users to download an appropriate edition of Citrix Receiver. It is important to note that no configuration will be accomplished via this download, so users will receive the first time use prompt to enter a server URL or email address. This option is likely to be utilized in a BYOD environment.

Selecting the appropriate deployment method is based on the type of Citrix Receiver selected. The following table should be referenced to help identify the appropriate deployment options for Citrix Receiver.

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Decision: Initial Configuration

Citrix Receiver must be configured in order to provide access to enterprise resources. The method of configuration varies by Citrix Receiver edition, the form factor of the device, and lastly the access method (local or remote) that is involved. Several methods may be viable for an organization. The method utilized is contingent on the resources (people, systems, time) available as well as larger organizational initiatives such as BYOD programs.

The following methods can be used to configure Citrix Receiver:

  • Email-based discovery - The latest releases of Citrix Receiver can be configured by entering an email address. Email based discovery requires Citrix StoreFront as well as an SRV DNS record which points to the FQDN of the StoreFront server.

Note: Any DNS platform should support email-based discovery, however only Windows DNS has been explicitly tested.

For remote access, NetScaler Gateway must be utilized with the corresponding SRV record in external DNS. A valid server certificate on the NetScaler Gateway appliance or StoreFront server must be present in order to enable email-based account discovery. This configuration assumes that the portion of the email address after the “@” is the DNS namespace that should be queried for this SRV record. This can be challenging for customers with different external and internal namespaces or email addresses that are different from DNS namespaces.

  • Group policy - Microsoft Group Policy can be used to configure Citrix Receiver. This can be done via start up scripts used to deploy Receiver by ensuring there is a value for the SERVER_LOCATION=Server_URL parameter or by using the ADMX/ADML template files included with the installation of Citrix Receiver to set the StoreFront Account List option in conjunction with another Receiver deployment method. Provide the URL of the server running Citrix StoreFront in the format https://baseurl/Citrix/storename/discovery.
  • Provisioning file - For environments running StoreFront, it is possible to provide users with a provisioning file that contains store information. Provisioning files are exported from the StoreFront console. The file is saved with a “*.cr” extension and can then be placed on a shared network resource, a Receiver for Web site, or other web based resource or emailed to users. The file can then be launched from an endpoint, which automatically configures Citrix Receiver to use the store(s). If users browse to the Receiver for Web site and select the “Activate” option under their username, this also automatically downloads this same “.cr” file and configure the Receiver client for users.
  • Manually - If allowed, it is usually possible to configure Citrix Receiver manually by entering the server URL. This method should be reserved for administrators or users that have advanced knowledge.
  • Studio - In addition to the above methods, in order to configure Receiver deployed on a virtual desktop or server image (within a XenDesktop or XenApp environment), it is possible to set the StoreFront address via the properties of the Delivery Group.

Decision: Updates

Citrix Receiver is in active development. As such, periodic updates are released that provide enhanced functionality or address user issues. As with any actively developed product, the latest version of these products should be deployed to the endpoints so that users benefit from the latest functionality and to maintain compliance with product support lifecycles. There are multiple methods available to update Citrix Receiver and, if applicable, associated plug-ins.

  • Enterprise software deployment - ESD tools provide an organization with direct control over the time/frequency of Receiver updates to managed devices. Additional thought must be given to updating unmanaged devices and endpoints outside of the corporate firewall.
  • Manual updates - When no automated solution is available, manual methods can be used to update Citrix Receiver. Whether deployed on Receiver for Web site, StoreFront, an internal Citrix Receiver site, or an external site, these options will require user involvement in updating Citrix Receiver. Due to the involved nature of manual updates coupled with the opportunity for a user mistake, this option should only be considered as a last resort.

Layer 2: The Access Layer

The second layer of the design methodology is the access layer, which is defined for each user group.

Creating an appropriate design for the access layer is an important part of the desktop virtualization process. This layer handles user validation through authentication and orchestrates access to all components necessary to establish a secure virtual desktop connection.

The access layer design decisions are based on the mobility requirements of each user group as well as the endpoint devices used.

Authentication

Getting access to resources is based on the user’s identity. Defining the authentication strategy takes into account the user’s entry point into the environment as well as how the user will authenticate.

Decision: Authentication Point

Before a user connects to a virtual resource, they must first authenticate. The place of authentication is often determined by the user group’s mobility requirements, which were defined during the user segmentation process. There are two authentication points available in XenDesktop 7.6:

  • StoreFront - Citrix StoreFront provides authentication and resource delivery services for Citrix Receiver, enabling centralized enterprise stores to deliver desktops, applications and other resources.
  • NetScaler Gateway - NetScaler Gateway is an appliance providing secure application access and granular application-level policy controls to applications and data while allowing users to work from anywhere.

The following table lists preferred authentication points according to user group mobility requirements:

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Authentication for user groups with a mobility requirement of remote or mobile may occur directly on StoreFront where required. For example, DMZ security policies may prohibit access from the NetScaler Gateway to the domain, which is required to support SmartCard client certificate authentication. Access to StoreFront for authentication may then be delivered via a NetScaler SSL_BRIDGE virtual server, which provides a conduit for https traffic. Typically, the virtual server would be hosted alongside a NetScaler Gateway on the same NetScaler configured to provide HDX Proxy access to the virtual desktop environment. Although such a use case may sometimes be necessary, the recommended best practice is to authenticate external users via NetScaler Gateway.

Decision: Authentication Policy

Once the authentication point has been identified, the type of authentication must be determined. The following options are the primary methods available:

  • StoreFront - Supports a number of different authentication methods, although not all are recommended depending on the user access method, security requirements and network location. Note that by default StoreFront authenticates users directly with Active Directory, not via XML as Web Interface did. StoreFront 3.0+ can be optionally configured to delegate authentication to XML if required (such as if the StoreFront servers are in a domain that does not trust the user domains).
    • User name and password - Requires users to logon directly to the site by entering a user name and password.
    • Domain pass-through - Allows pass-through of domain credentials from users' devices. Users authenticate to their domain-joined Windows computers and are automatically logged on when they access their stores.
    • NetScaler Gateway pass-through - Allows pass-through authentication from NetScaler Gateway. Users authenticate to NetScaler Gateway and are automatically logged on when they access their stores.
    • Smart card - Allows users to authenticate using smart cards and PINs through Citrix Receiver for Windows and NetScaler Gateway. To enable smart card authentication, user accounts must be configured either within the Microsoft Active Directory domain containing the StoreFront servers or within a domain that has a direct two-way trust relationship with the StoreFront server domain. Multi-forest deployments involving one-way trust or trust relationships of different types are not supported.
    • Anonymous - Allow users to access applications and desktops without presenting credentials to StoreFront or Citrix Receiver. Local anonymous accounts are created on demand on the Server VDA when sessions are launched. This requires a StoreFront store configured for authenticated access, a Server OS based VDA, and a XenApp 7.6 (or later) Delivery Group configured for unauthenticated users.
  • NetScaler Gateway - The NetScaler Gateway supports several authentication methods. The list below includes those primarily used in virtual desktop environments. Each may be used individually, but are often combined to provide multi-factor authentication.
    • LDAP - The lightweight directory access protocol (LDAP) is used to access directory information services such Microsoft Active Directory. NetScaler Gateway uses LDAP to authenticate users and extract their group membership information.
    • RADIUS (token) - Remote authentication dial in user service (RADIUS) is a UDP based network security protocol that provides authentication, authorization and accounting. A network access server (NetScaler Gateway in this case) forwards credentials to a RADIUS server that can either check the credentials locally, or check them against a directory service. The RADIUS server could then accept the connection, reject the connection, or challenge and request a second form of authentication such as a token.
    • Client certificate - Users logging on to a NetScaler Gateway virtual server can also be authenticated based on the attributes of a client certificate presented to the virtual server. Client certificates are usually disseminated to users in the form of smartcards or common access cards (CACs) that are read by a reader attach to each user’s device.

The authentication type for a user group is often determined based on security requirements as well as the authentication point used. The following table helps define the appropriate solution for each user group based on the level of security required:

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StoreFront

Citrix StoreFront authenticates users to XenApp and XenDesktop resources. StoreFront enumerates and aggregates available desktops and applications into a single interface that users access through Citrix Receiver for Windows, iOS, Android, or the StoreFront web site.

Decision: High Availability

If the server hosting StoreFront is unavailable, users cannot start new virtual desktops, published applications or manage their subscriptions. Therefore, deploy at least two StoreFront servers to prevent this component from becoming a single point of failure. By implementing a load balancing solution, users will not experience an interruption in their service. Options include:

  • Hardware load balancing - An intelligent appliance, which is capable of verifying the availability of the StoreFront service and actively load balance user requests appropriately. Citrix NetScaler is a great example of a hardware load balancer. Citrix NetScaler is an ideal load balancer, coming pre-configured with StoreFront health checks.
  • DNS round robin - Provides rudimentary load balancing across multiple servers without performing any checks on availability. If a StoreFront server becomes unavailable, DNS round robin would still route users to the failed server. Because of this, DNS round robin is not recommended by Citrix.
  • Windows network load balancing – A Windows service capable of performing rudimentary checks to verify the server is available but cannot determine the status of individual services. This can cause users to be forwarded to StoreFront servers which are not able to process new requests. The user would then not be able to access applications or desktops.

Decision: Delivery Controller Reference

To provide users with desktops and applications, StoreFront must be configured with the IP address or DNS name of at least one Controller in each XenDesktop and XenApp site. For fault tolerance, multiple controllers should be entered for each site and/or farm specified. By default, StoreFront treats a list of servers in failover order (active/passive).

For large deployments or environments with a high logon load an active distribution of the user load (active/active) is recommended. This can be achieved by means of a load balancer with built-in XML monitors, such as Citrix NetScaler or by configuring StoreFront to load balance the list of Controllers instead of treating them as an ordered list.

Decision: Beacons

Citrix Receiver uses beacons (websites) to identify whether a user is connected to an internal or external network. Internal users are connected directly to StoreFront for authentication while external users are connected via Citrix NetScaler Gateway. It is possible to control what a user sees by restricting applications due to which beacon they have access to.

The internal beacon should be a site that is not resolvable externally. By default, the internal beacon is the StoreFront base URL. This will have to be adjusted if the same external and internal URL is configured. The external beacon can be any external site that produces an http response. Citrix Receiver continuously monitors the status of network connections (for example, link up, link down or change of the default gateway). When a status change is detected, Citrix Receiver first verifies that the internal beacon points can be accessed before moving on to check the accessibility of external beacon points. StoreFront provides Citrix Receiver with the http(s) addresses of the beacon points during the initial connection/configuration download process and provides updates as necessary.

It is necessary to specify at least two highly available external beacons that can be resolved from public networks.

Decision: Resource Presentation

By default, StoreFront allows users to choose (subscribe) to the resources they want to regularly use after they logon (favorites). This approach, deemed “Self-Service,” allows users to restrict the resources that they see on their home screen to the ones that they use on a regular basis. The resources chosen by every user for each store are recorded by the subscription store service and stored locally on each StoreFront server (synced automatically between servers in the same server group) so that they can be displayed on the Citrix Receiver home screen from any device that the user connects from. Although by default subscriptions are per store and per server group, administrators can configure two stores within a server group to share a subscription database and/or sync subscriptions between two identically named stores in two separate server groups on a defined schedule if required.

Administrators should determine which applications should always be displayed to users on their home screen or the featured tab. In general, these applications are common applications such as the Microsoft Office Suite and any other applications that every user in an environment may need. StoreFront can filter/present these resources using Keywords defined within the published application properties Description field.

The following table explores the Keyword options:

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Decision: Scalability

The number of Citrix Receiver users supported by a single StoreFront server depends on the resources assigned and level of user activity. Note that Receiver for Web users will consume more RAM on average than native Receiver users, but a minimum of 4 GB of RAM is recommended per StoreFront server in all cases as a baseline. Additionally, more sites/farms enumerated per store will increase both CPU utilization and server response time, with XenApp IMA farms having a greater scalability impact than XenApp/XenDesktop FMA site.

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Tests have shown diminishing returns after a single StoreFront deployment grows beyond 3-4 StoreFront nodes with a maximum of 5-6 servers supported in a single server group.

NetScaler Gateway

Selection of the network topology is central to planning the remote access architecture to ensure that it can support the necessary functionality, performance, and security. The design of the remote access architecture should be completed in collaboration with the security team to ensure adherence to corporate security requirements. There are two primary topologies to consider, each of which provides increasing levels of security:

Decision: Topology

Selection of the network topology is central to planning the remote access architecture to ensure that it can support the necessary functionality, performance and security. The design of the remote access architecture should be completed in collaboration with the security team to ensure adherence to corporate security requirements. There are two primary topologies to consider, each of which provides increasing levels of security:

  • 1-Arm (normal security) - With a 1-arm topology, the NetScaler Gateway utilizes one physical or logical bonded interface, with associated VLAN and IP subnet, to transport both frontend traffic for users and backend traffic for the virtual desktop infrastructure servers and services.
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  • 2-Arm (high security) - With a 2-arm topology, the NetScaler Gateway utilizes two or more physically or logically bonded interfaces, with associated VLANS and IP subnets. Transport of the frontend traffic for users is directed to one of these interfaces. The frontend traffic is isolated from backend traffic, between the virtual desktop infrastructure servers and services, which is directed to a second interface. This allows the use of separate demilitarized zones (DMZs) to isolate frontend and backend traffic flows along with granular firewall control and monitoring.
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Decision: High Availability

If the NetScaler Gateway is unavailable, remote users will not be able to access the environment. Therefore, at least two NetScaler Gateway hosts should be deployed to prevent this component from becoming a single point of failure.

When configuring NetScaler Gateway in a high availability (active/passive) pair, the secondary NetScaler Gateway monitors the first appliance by sending periodic messages, also called a heartbeat message or health check, to determine if the first appliance is accepting connections. If a health check fails, the secondary NetScaler Gateway tries the connection again for a specified amount of time until it determines that the primary appliance is not working. If the secondary appliance confirms the health check failure, the secondary NetScaler Gateway takes over for the primary NetScaler Gateway.

Note that in firmware 10.5 and above, clustering is also possible with multiple NetScaler Gateway instances to provide high availability, although support for spotted versus stripped configurations varies by firmware and Gateway configuration (full SSL VPN versus ICA proxy). (http://docs.citrix.com/en-us/netscaler/11-1/clustering/cluster-features-supported.html)

Decision: Platform

In order to identify an appropriate NetScaler platform to meet project requirements, the key resource constraints must be identified. Since all remote access traffic will be secured using the secure sockets layer (SSL), transported by Hypertext Transfer Protocol (HTTP) in the form of HTTPs, there are two resource metrics that should be targeted:

  • SSL throughput – The SSL throughput is the gigabits of SSL traffic that may be processed per second (Gbps).
  • SSL transactions per second (TPS) – The TPS metric identifies how many times per second an Application Delivery Controller (ADC) may execute an SSL transaction. The capacity varies primarily by the key length required. TPS capacity is primarily a consideration during the negotiation phase when SSL is first setup and it is less of a factor in the bulk encryption / decryption phase, which is the majority of the session life. While TPS is an important metric to monitor, field experience has shown that SSL throughput is the most significant factor in identifying the appropriate NetScaler Gateway.

The SSL bandwidth overhead average is often considered negligible relative to the volume of virtual desktop traffic and is not typically accounted for as part of required SSL throughput. However, making provisions for SSL bandwidth will help ensure the total throughput estimated is sufficient. The fixed bandwidth added to packet headers can vary according to the encryption algorithms used and the overall percentage of bandwidth may vary widely according to packet size. Ideally, the overhead should be measured during a proof of concept or pilot. However, in the absence of such data incrementing the workload bandwidth by 2% is a reasonable rule of thumb. Therefore, to determine the SSL throughput required by a NetScaler platform, multiply the maximum concurrent bandwidth for a datacenter by 1.02:

𝑆𝑆𝐿 𝑇ℎ𝑟𝑜𝑢𝑔ℎ𝑝𝑢𝑡=𝑀𝑎𝑥𝑖𝑚𝑢𝑚 𝐶𝑜𝑛𝑐𝑢𝑟𝑟𝑒𝑛𝑡 𝐵𝑎𝑛𝑑𝑤𝑖𝑑𝑡ℎ ∗ 1.02

For example, assuming 128Mbps maximum concurrent bandwidth, the appropriate NetScaler model can be determined as follows:

~130𝑀𝑏𝑝𝑠=128𝑀𝑏𝑝𝑠 ∗ 1.02

The SSL throughput value should be compared to the throughput capabilities of various NetScaler platforms to determine the most appropriate one for the environment. There are three main platform groups available, each of which provides broad scalability options.

  • VPX - A NetScaler VPX device provides the same full functionality as hardware NetScaler. However, NetScaler VPXs can leverage ‘off the shelf’ servers for hosting and are suitable for small to medium sized environments. Typically, organizations create a baseline cap for the VPX instances at 500 users.
  • MDX - A NetScaler MDX is the hardware version of the NetScaler devices. The MDX device is more powerful than the virtual NetScaler and can support network optimizations for larger scale enterprise deployments, particularly when SSL offload will be configured as this is done in software on the VPX versus dedicated SSL chips on the MPX.
  • SDX - A NetScaler SDX is a blend between the virtual and physical NetScaler devices. An SDX machine is a physical device capable of hosting multiple virtual NetScaler devices. This consolidation of devices aids with reducing required shelf space and device consolidation. NetScaler SDXs are suitable for handling network communications for large enterprise deployments and/or multi-tenant hosting providers.

SSL throughput capabilities of the NetScaler platforms may be found in the Citrix NetScaler data sheet. Therefore, based on the example calculation above, a NetScaler MPX 5550 appliance would be sufficient to handle the required load. However, actually scalability will depend on security requirements. NetScaler SSL throughput decreases with the use of increasingly complex encryption algorithms and longer key lengths. Also, this calculation represents a single primary NetScaler. At a minimum, N+1 redundancy is recommended which would call for an additional NetScaler of the identical platform and model.

Note: The Citrix NetScaler data sheet typically represents throughput capabilities under optimal conditions for performance. However, performance is directly affected by security requirements. For example, if the RC4 encryption algorithm and a 1k key length are used, a VPX platform may be able to handle more than 500 HDX proxy connections. However, if a 3DES encryption algorithm and 2k key length are used (which are becoming more common), the throughput may be halved.

Decision: Pre-Authentication Policy

An optional pre-authentication policy can be applied to user groups with NetScaler Gateway as their authentication point. Pre-authentication policies limit access to the environment based on whether the endpoint meets certain criteria through Endpoint Analysis (EPA) Scans.

Pre-authentication access policies can be configured to test antivirus, firewall, operating system, or even registry settings. These policies can be used to prevent access entirely or can be used by XenDesktop to control session features such as clipboard mapping, printer mapping and even the availability of specific applications and desktops. For example, if a user device does not have antivirus installed, a filter can be set to hide sensitive applications.

The following figure provides an overview of how multiple policies can be used to customize the features of a virtualization resource:

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Decision: Session Policy

User groups with NetScaler Gateway as their authentication point must have corresponding session policies defined. Session policies are used to define the overall user experience post-authentication.

Organizations create sessions policies based on the type of Citrix Receiver used. For the purpose of session policy assignment, devices are commonly grouped as either non-mobile (such as Windows, Mac and Linux OS based), or mobile (such as iOS or Android). Therefore a decision on whether to provide support for mobile devices, non-mobile devices, or both should be made based on client device requirements identified during the assess phase.

To identify devices session policies, include expressions such as (Configuring Session Policies and Profiles for App Controller and StoreFront):

  • Mobile devices - The expression is set to REQ.HTTP.HEADER User-Agent CONTAINS CitrixReceiver which is given a higher priority than the non-mobile device policy to ensure mobile devices are matched while non-mobile devices are not.
  • Non-mobile devices – The expression is set to ns_true which signifies that it should apply to all traffic that is sent to it.

An alternative use of session policies is to apply endpoint analysis expressions. These session policies are applied post authentication yet mimic the previously mentioned pre-authentication policies. Use of session policies is an option to provide a fallback scenario to endpoints that do not meet full security requirements such read-only access to specific applications.

Decision: Session Profile

Each session policy must have a corresponding session profile defined. The session profile defines details required for the user group to gain access to the environment. There are two primary forms of session profiles that determine the access method to the virtual desktop environment:

  • SSLVPN - Users create a virtual private network and tunnel all traffic configured by IP addresses through the internal network. The user’s client device is able to access permitted intranet resources as if it were on the internal network. This includes XenDesktop sites and any other internal traffic such as file shares or intranet websites. This is considered a potentially less secure access method since network ports and routes to services outside of the virtual desktop infrastructure may be opened leaving the enterprise susceptible to risks that may come with full VPN access. These risks may include denial of service attacks, attempts at hacking internal servers, or any other form of malicious activity that may be launched from malware, trojan horses, or other viruses via an Internet based client against vulnerable enterprise services via routes and ports.

    Another decision to consider when SSLVPN is required is whether to enable split tunneling for client network traffic. By enabling split tunneling, client network traffic directed to the intranet by Citrix Receiver may be limited to routes and ports associated with specific services. By disabling split tunneling, all client network traffic is directed to the intranet, therefore both traffic destined for internal services as well as traffic destined for the external services (Internet) traverses the corporate network. The advantage of enabling split tunneling is that exposure of the corporate network is limited and network bandwidth is conserved. The advantage of disabling split tunneling is that client traffic may be monitored or controlled through systems such as web filters or intrusion detection systems.
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  • HDX proxy - With HDX Proxy, users connect to their virtual desktops and applications through the NetScaler Gateway without exposing internal addresses externally. In this configuration, the NetScaler Gateway acts as a micro VPN and only handles HDX traffic. Other types of traffic on the client’s endpoint device, such as private mail or personal Internet traffic do not use the NetScaler Gateway.

    Based on the endpoint and Citrix Receiver used, a decision must be made as to whether this method is supported for each user group. HDX Proxy is considered a secure access method for remote virtual desktop access since only traffic specific to the desktop session is allowed to pass through to the corporate infrastructure. Most Citrix Receivers support HDX Proxy and it is the preferred method:
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Decision: Preferred Datacenter

Enterprises often have multiple active datacenters providing high availability for mission critical applications. Some virtual desktops or applications may fall into that category while others may only be accessed from a specific preferred datacenter. Therefore, the initial NetScaler Gateway that a user authenticates to in a multi-active datacenter environment may not be within the preferred datacenter corresponding to the user’s VDI resources. StoreFront is able to determine the location of the user’s assigned resources and direct the HDX session to those resources; however, the resulting path may be sub-optional (WAN connection from the NetScaler Gateway to the virtual desktop/application resources as opposed to LAN connection).

There are static and dynamic methods available to direct HDX sessions to their virtual desktop resources in their primary datacenter. The decision regarding which method to select should be based on the availability of technology to dynamically assign sites links such as Global Server Load Balancing (GSLB) along with the network assessment of intranet and Internet bandwidth as well as Quality of Service (QoS) capabilities.

Note: For more information on configuring the static and dynamic methods of GSLB, please refer to Citrix Product Documentation - Configuring GSLB for Proximity.

  • Static
    • Direct - The user can be given a FQDN mapped to an A record that is dedicated to the primary datacenter NetScaler Gateway(s) allowing them to access their virtual desktop directly wherever they are in the world. This approach eliminates a layer of complexity added with dynamic allocation. However, it also eliminates fault tolerance options such as the ability to access the virtual desktop through an alternative intranet path when a primary datacenter outage is limited to the access infrastructure.
  • Dynamic
    • Intranet - For most dynamic environments, the initial datacenter selected for authentication is the one closest to the user. Protocols such as GSLB dynamic proximity calculate the least latency between the user’s local DNS server and the NetScaler Gateway. Thereafter, by default, the HDX session is routed through the same NetScaler Gateway to whichever datacenter is hosting the user’s virtual desktops and applications. The advantage of this approach is that the majority of the HDX session would traverse the corporate WAN where quality of service may be used.
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  • Internet - Alternatively, the HDX session can be re-routed through an alternate NetScaler Gateway proximate to the backend VDI desktop / XenApp server, resulting in most of the HDX session travelling over the Internet. For example, a user with a dedicated desktop in the United Stated, traveling in Europe may be directed to a NetScaler Gateway hosted in a European datacenter based on proximity. However, when the user launches their desktop, an HDX connection will be established to the virtual desktop via a NetScaler Gateway hosted in the preferred datacenter in the United States.

    This conserves WAN network usage (at the cost of QoS) and is recommended in cases where the user’s Internet connection may provide a more reliable experience than the corporate WAN.
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Some customers will used a combination of these methods, such as geo-specific dymanic URLs such that fault tolerance is provided within a geographic area (such as North America) without incurring the complexity of global GSLB.

Layer 3: The Resource Layer

The resource layer is the third layer of the design methodology and the final layer focused specifically on the user groups.

The overall user acceptance of the solution is defined by the decisions made within the resource layer. Profiles, printing, applications and overall desktop image design play a pivotal role in how well the desktop is aligned with the user group’s requirements, which were identified within the assess phase.

User Profiles

A user’s profile plays a critical role in delivering a consistently positive experience within a virtual desktop or virtual application scenario. Even a well-designed virtual desktop solution can fail if users are frustrated due to lengthy logon times or lost settings.

The user profile solution chosen must align with the personalization characteristics of the user group captured during the assess phase as well as the VDI model selected.

Decision: Profile Type

This section provides an overview on the different profile types available and provides guidance on the optimal user profile for each VDI model.

  • Local profiles - Local profiles are stored on each server or desktop operating system and are initially created based on the default user profile. Therefore, a user accessing these resources would create an independent profile on each system. Users are able to retain changes to their local profile on each individual system, but changes are only accessible for future sessions on that system. Local profiles require no configuration; if a user logging into a server or desktop operating system does not have a profile path administratively defined, a local profile is created by default.
  • Roaming profiles - Roaming profiles are stored in a centralized network repository for each user. Roaming profiles differ from local profiles in that the information in the profile (whether it is a printer, a registry setting, or a file stored in the documents folder) can be made available to user sessions accessed from all systems in the environment. Configuring a user for a roaming profile requires an administrator to designate the user’s profile path (for virtual desktops) or terminal server profile path to a particular network share. The first time the user logs on to a server or desktop operating system, the default user profile is used to create the user’s roaming profile. During logoff, the profile is copied to the administrator-specified network location.
  • Mandatory profiles - Mandatory profiles are typically stored in a central location for many users. However, the user’s changes are not retained at logoff. Configuring a user for a mandatory profile requires an administrator to create a mandatory profile file (NTUSER.MAN) from an existing roaming or local profile and assign users’ with a terminal services profile path. This can be achieved by means of Microsoft Group Policy, customizing the user properties in Active Directory or Citrix Profile Management.
  • Hybrid profiles - Hybrid profiles combine a robust profile core (a mandatory profile or a local default profile) with user specific registry keys or files that are merged during logon. This technique enables administrators to tightly control which changes are retained and to keep the user profiles small in size. Furthermore, hybrid profiles address the last write wins issue using mature queuing techniques that automatically detect and prevent simultaneous writes that could potentially overwrite changes made in another session. Thus minimizing, user frustration resulting from lost profile changes when accessing multiple servers or virtual desktops simultaneously. In addition, they capture and record only the changes within the profile, rather than writing the entire profile at logoff. A good example of a hybrid profile solution is Citrix Profile Management, which will be discussed in detail within this chapter.

The following table compares the capabilities of each profile type:

In order to select the optimal profile type for each user group, it is important to understand their personalization requirements in addition to the FlexCast model assigned.

The following table provides recommendations on selecting the appropriate user profile type based on VDI resource:

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Decision: Folder Redirection

Redirecting special folders can supplement any of the described profile types. While redirecting profile folders, such as user documents and favorites, to a network share is a good practice to minimize profile size, architects need to be aware that applications may frequently read and write data to profile folders such as AppData, causing potential issues with file server utilization and responsiveness. It is important to thoroughly test profile redirection before implementation in production to avoid these issues. Therefore, it is important to research profile read / write activities and to perform a pilot before moving to production. Microsoft Outlook is an example of an application that regularly performs profile read activities, as the user signature is read from the user profile every time an email is created.

The following table provides general recommendations to help identify the appropriate folders to redirect:

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Decision: Folder Exclusion

Excluding folders from being persistently stored as part of a roaming of hybrid profile can help to reduce profile size and logon times. By default Windows excludes the AppData\Local and AppData\LocalLow folders, including all subfolders, such as History, Temp and Temporary Internet Files. In addition, the downloads and saved games folders should also be excluded. All folders that are redirected should be excluded from the profile solution.

Decision: Profile Caching

Local caching of roaming or hybrid user profiles on a server or virtual desktop is default Windows behavior and can reduce login times and file server utilization / network traffic. With profile caching, the system only has to download changes made to the profile. The downside of profile caching is that it can consume significant amounts of local disk storage on multi-user systems, such as a hosted shared desktop hosts.

In certain VDI models and configurations, the VDI resource is reset to a pristine state. Having locally cached profiles be deleted upon logoff is an unnecessary consumption of resources. Based on this, the leading recommendation is to not deleting locally cached profiles for the following VDI models:

  • Hosted Personal Desktops
  • Hosted Pooled Desktops - only in situations where a reboot occurs after logoff.
  • Local VM Desktops
  • Remote PC Access

Configuring the “Delay before deleting cached profiles” Citrix policy sets an optional extension to the delay before locally cached profiles are deleted at logoff. Extending the delay is useful if a process keeps files or the user registry hive open during logoff. This can also reduce logoff times for large profiles.

Decision: Profile Permissions

For security reasons, administrators, by default, cannot access user profiles. While this level of security may be required for organizations that deal with very sensitive data, it is unnecessary for most environments and can complicate operations and maintenance. Therefore, consider enabling the “Add the Administrators security group to roaming user profiles” policy setting. The configuration of this policy should be aligned with the security characteristics of the user groups captured during the assess phase. For more information on the permissions required for the file share hosting user profiles and data, please refer to Microsoft TechNet - Deploying Roaming Profiles.

Decision: Profile Path

Determining the network path for the user profiles is one of the most significant decisions during a user profile design process. In general it is strongly recommended to leverage a redundant and high performance file server or NAS device.

There are four topics that must be considered for the profile share:

  • Performance - File server performance will affect logon and logoff times, and depending on other decisions such as redirected folders and profile streaming, can impact the user’s experience within the session. For large virtual desktop infrastructures, a single file server cluster may not be sufficient to handle periods of peak activity. In order to distribute the load across multiple file servers, the file server address and share name will need to be adjusted.
  • Location - User profiles are transferred over the network by means of the SMB protocol, which does not perform well on high-latency network connections. Furthermore, WAN connections are typically bandwidth constrained, which can add additional delay to the profile load process. Therefore, the file server should be located in close proximity to the servers and virtual desktops to minimize logon times.
  • Operating system platforms - User profiles have a tight integration with the underlying operating system and it is not supported to reuse a single user profile on different operating systems or different platforms like 64-Bit (x64) and 32-Bit (x86). For more information, please refer to the Microsoft knowledge base article KB2384951 – Sharing 32 and 64-bit User Profiles. Windows 2008 and Windows Vista introduced a new user profile structure, which can be identified by .V2 profile directory suffix, which makes older user profiles incompatible with newer operating systems such as Windows 2012, 7 and 8. In order to ensure that a separate profile is used per platform, the profile directory has to be adapted.
  • Indexing capabilities - To take full advantage of Windows Search functionality on a user’s redirected data, Windows file servers that index the user’s data must be used, as opposed to a share on a NAS appliance. This is important for use cases that are heavily dependent on Windows Search or are especially sensitive to perception of slowness or latency.

There are two methods that can be used to address these challenges that are based on Windows built-in technology:

  • User object - For every user object in Active Directory, an individual profile path, which contains file server name and profile directory, can be specified. Since only a single profile path can be specified per user object, it is not possible to ensure that a separate profile is loaded for each operating system platform.
  • Computer group policy or system variables - The user profile path can also be configured by means of computer specific group policies or system variables. This enables administrators to ensure that a user profile is dedicated to the platform. Since computer specific configurations affect all users of a system, all user profiles will be written to the same file server. To load balance user profiles across multiple servers dedicated XenDesktop delivery groups have to be created per file server.

Note: Microsoft does not support DFS-N combined with DFS-R for actively used user profiles. For more information, please refer to the Microsoft articles:

When using Citrix Profile Management, a third option is available to address these challenges:

  • User object attributes and variables - Citrix Profile Management enables the administrator to configure the profile path by means of a computer group policy using attributes of the user object in Active Directory to specify the file server dynamically. In order to achieve this, three steps are required:
    1. Create a DNS alias (for example, fileserver1) that refers to the actual file server
    2. Populate an empty LDAP attribute of the user object (for example, l or UID) with the DNS Alias
    3. Configure Citrix Profile Management by means of GPO to use a profile path that refers to the LDAP attribute (for example, If attribute UID is used the profile path becomes \\#UlD#\Profiles\profiledirectory)

In addition, Citrix Profile Management automatically populates variables to specify the profile path dynamically based on the operating system platform. Valid profile management variables are:

  • !CTX_PROFILEVER! - Expands to v1 or v2 depending on the profile version.
  • !CTX_OSBITNESS! - Expands to x86 or x64 depending on the bit-level of the operating system.
  • !CTX_OSNAME! - Expands to the short name of the operating system, for example Win7

By combining both capabilities of Citrix Profile Management, a fully dynamic user profile path can be created, which can be load balanced across multiple file servers and ensure profiles of different operating system platforms are not mixed. An example of a fully dynamic user profile path is shown below:

\\#UID#\profiles$\%USERNAME%.%USERDOMAIN%\!CTX_OSNAME!!CTX_OSBITNESS!

Decision: Profile Streaming

Note: The following design decision only applies to those environments that use Citrix Profile Management.

With user profile streaming, files and folders contained in a profile are fetched from the user store (file server) to the local computer when a user accesses them. During the logon process, Citrix Profile Management immediately reports that the profile load process has completed reducing profile load time to almost zero.

Citrix recommends enabling profile streaming for all scenarios. If it is desired to keep a local cached copy of the user profile for performance reasons, it is recommended to enable the “Always Cache” setting and configure a size of 0. This ensures that the user profile is downloaded in the background and enables the system to use this cached copy going forward.

Note: Profile streaming is not required and does not work with the personal vDisk feature of Citrix XenDesktop. Even if explicitly enabled by means of Group Policy, the profile streaming setting is automatically disabled.

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Decision: Active Write Back

Note: The following design decision only applies to those environments that use Citrix Profile Management.

By enabling the active write back feature, Citrix Profile Manager detects when an application has written and closed a file and copies the file back to the network copy of the profile during idle periods. In scenarios where a single user leverages multiple virtual desktops or hosted shared desktops simultaneously, this feature can be tremendously beneficial. However, Citrix Profile Management does not copy any registry changes back to the network, except during an ordered logoff. As such, there is a risk that the registry and files may get out of alignment on non-persistent systems, where locally cached profile information is wiped upon reboot. Therefore, it is recommended to disable active write back functionality for non-persistent scenarios.

Decision: Configuration Approach

Note: The following design decision only applies to those environments that use Citrix Profile Management.

Citrix Profile Management can be configured by means of an “.ini” file, Microsoft Group Policy and Citrix Policy (Citrix Profile Management 5.0 and newer). While each option offers the same configuration settings, Group Policy is recommended because it allows administrators to perform Windows and Citrix profile configurations from a single point, minimizing the tools necessary for profile management.

Note: With Citrix Profile Management 5.0 and newer, the desktop type is automatically detected and Citrix Profile Management policies set accordingly. For more information, please refer to Citrix eDocs – How automatic configuration works.

Decision: Replication

While having an active/active datacenter on a network level is easily accomplished with GSLB, the replication of user data makes having a fully active/active deployment complex in most situations. To have an active/active configuration where users are not statically assigned to a specific datacenter, will require users to have no form of personalization requirements. This will limit the user’s ability to make any configuration changes and will not allow them to create any documents or persistent data. The exception to this is when a high-spee,d low latency connection such as dark fibre is available between datacenters. This will let resources in both locations can point to the same file server allowing for a true active/active solution. Also, an active/active configuration can be accomplished when applications are used that rely solely on a backend database that is actively replicated between datacenters and do not store any data in the user profile.

For redundancy and failover purposes, user data such as Windows profiles and documents should be synchronized between datacenters. Although it is recommended to replicate user data between datacenters, the replication would be an active/passive configuration. This means the data can only be actively consumed from a single datacenter. The reason for this limitation is the distributed file locking method inside Windows that only allows a single user to actively write to a file. Therefore, active/active replication of user data is not supported. Any supported configuration consists of a one-way replication of data that is active in a single datacenter at any point in time.

For example, the figure below describes a scenario where user data is passively replicated from Datacenter A to Datacenter B. In this example, File Server A is the primary location for user data in Datacenter A and File Server B is the primary location in Datacenter B. One-way replication of the user data occurs for each fileserver to allow for the user data to be available in the opposite datacenter if a failover occurs. Replication technologies such as Microsoft DFS can be configured to mirror user profiles and documents to a file server in another datacenter. DFS Namespaces can also be used to have a seamless path for the location of the user data. However, implementing a replication solution like this requires an administrator familiar with Microsoft DFS and user profiles.

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Policies

Policies provide the basis to configure and fine tune XenApp and XenDesktop environments, allowing organizations to control connection, security and bandwidth settings based on various combinations of users, devices or connection types.

When making policy decisions it is important to consider both Microsoft and Citrix policies to ensure that all user experience, security and optimization settings are considered. For a list of all Citrix-related policies, please refer to the Citrix Policy Settings Reference.

Decision: Preferred Policy Engine

Organizations have the option to configure Citrix policies via Citrix Studio or through Active Directory group policy using Citrix ADMX files, which extend group policy and provide advanced filtering mechanisms.

Using Active Directory group policy allows organizations to manage both Windows policies and Citrix policies in the same location, and minimizes the administrative tools required for policy management. Group policies are automatically replicated across domain controllers, protecting the information and simplifying policy application.

Citrix administrative consoles should be used if Citrix administrators do not have access to Active Directory policies. Architects should select one of the above two methods as appropriate for their organization’s needs and use that method consistently to avoid confusion with multiple Citrix policy locations.

It is important to understand how the aggregation of policies, known as policy precedence flows in order to understand how a resultant set of policies is created. With Active Directory and Citrix policies, the precedence is as follows:

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Policies from each level are aggregated into a final policy that is applied to the user or computer. In most enterprise deployments, Citrix administrators do not have rights to change policies outside their specific OUs, which will typically be the highest level for precedence. In cases where exceptions are required, the application of policy settings from higher up the OU tree can be managed using “block inheritance” and “no override” settings. Block inheritance stops settings from higher-level OUs (lower precedence) from being incorporated into the policy. However, if a higher-level OU policy is configured with no override, the block inheritance setting will not be applied. Given this, care must be taken in policy planning, and available tools such as the “Active Directory Resultant Set of Policy” tool or the “Citrix Group Policy Modeling” wizard should be used to validate the observed outcomes with the expected outcomes.

Note: Some Citrix policy settings, if used, need to be configured through Active Directory group policy, such as Controllers and Controller registration port, as these settings are required for VDAs to register.

Decision: Policy Integration

When configuring policies, organizations often require both Active Directory policies and Citrix policies to create a completely configured environment. With the use of both policy sets, the resultant set of policies can become confusing to determine. In some cases, particularly with respect to Windows Remote Desktop Services (RDS) and Citrix policies, similar functionality can be configured in two different locations. For example, it is possible to enable client drive mapping in a Citrix policy and disable client drive mapping in a RDS policy. The ability to use the desired feature may be dependent upon the combination of RDS and Citrix policy. It is important to understand that Citrix policies build upon functionality available in Remote Desktop Services. If the required feature is explicitly disabled in RDS policy, Citrix policy will not be able to affect a configuration as the underlying functionality has been disabled.

In order to avoid this confusion, it is recommended that RDS policies only be configured where required and there is no corresponding policy in the XenApp and XenDesktop configuration, or the configuration is specifically needed for RDS use within the organization. Configuring policies at the highest common denominator will simplify the process of understanding resultant set of policies and troubleshooting policy configurations.

Decision: Policy Scope

Once policies have been created, they need to be applied to groups of users and/or computers based on the required outcome. Policy filtering provides the ability to apply policies against the requisite user or computer groups. With Active Directory based policies, a key decision is whether to apply a policy to computers or users within site, domain or organizational unit (OU) objects. Active Directory policies are broken down into user configuration and computer configuration. By default, the settings within the user configuration apply to users who reside within the OU at logon, and settings within the computer configuration are applied to the computer at system startup, and will affect all users who logon to the system. One challenge of policy association with Active Directory and Citrix deployments revolves around three core areas:

  • Citrix environment specific computer policies - Citrix servers and virtual desktops often have computer policies that are created and deployed specifically for the environment. Applying these policies is easily accomplished by creating separate OU structures for the servers and the virtual desktops. Specific policies can then be created and confidently applied to only the computers within the OU and below and nothing else. Based upon requirements, virtual desktops and servers may be further subdivided within the OU structure based on server roles, geographical locations or business units.
  • Citrix specific user policies -– When creating policies for XenApp and XenDesktop there are a number of policies specific to user experience and security that are applied based on the user’s connection. However, the user’s account could be located anywhere within the Active Directory structure, creating difficulty with simply applying user configuration based policies. It is not desirable to apply the Citrix specific configurations at the domain level as the settings would be applied to every system any user logs on to. Simply applying the user configuration settings at the OU where the Citrix servers or virtual desktops are located will also not work, as the user accounts are not located within that OU. The solution is to apply a loopback policy, which is a computer configuration policy that forces the computer to apply the assigned user configuration policy of the OU to any user who logs onto the server or virtual desktop, regardless of the user’s location within Active Directory. Loopback processing can be applied with either merge or replace settings. Using replace overwrites the entire user GPO with the policy from the Citrix server or virtual desktop OU. Merge will combine the user GPO with the GPO from the Citrix server or desktop OU. As the computer GPOs are processed after the user GPOs when merge is used, the Citrix related OU settings will have precedence and be applied in the event of a conflict. For more information, please refer to the Microsoft TechNet article - Understand User Group Policy Loopback Mode.
  • Active Directory policy filtering - In more advanced cases, there may be a need to apply a policy setting to a small subset of users such as Citrix administrators. In this case, loopback processing will not work, as the policy should only be applied to a subset of users, not all users who logon to the system. Active Directory policy filtering can be used to specify specific users or groups of users to which the policy is applied. A policy can be created for a specific function, and then a policy filter can be set to apply that policy only to a group of users such as Citrix administrators. Policy filtering is accomplished using the security properties of each target policy.

Citrix policies created using Citrix Studio have specific filter settings available, which may be used to address policy-filtering situations that cannot be handled using group policy. Citrix policies may be applied using any combination of the following filters:

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Note: Citrix policies in XenDesktop 7.x provide a merged view of settings that apply at the user and computer level. In table 24, the Scope column identifies whether the specified filter applies to user settings, computer settings, or both.

Decision: Baseline Policy

A baseline policy should contain all common elements required to deliver a high-definition experience to the majority of users within the organization. A baseline policy creates the foundation for user access, and any exceptions that may need to be created to address specific access requirements for groups of users. It should be comprehensive to cover as many use cases as possible and should have the lowest priority, for example 99 (a priority number of “1” is the highest priority), in order to create the simplest policy structure possible and avoid difficulties in determining the resultant set of policies. The unfiltered policy set provided by Citrix as the default policy may be used to create the baseline policy as it is applied to all users and connections. In the baseline configuration, all Citrix policy settings should be enabled, even those that will be configured with the default value, in order to explicitly define desired/expected behavior, and to avoid confusion should default settings change over time.

Citrix Policy templates can be used to configure Citrix policies to effectively manage the end-user experience within an environment and can serve as an initial starting point for a baseline policy. Templates consist of pre-configured settings that optimize performance for specific environments or network conditions. The built-in templates included in XenDesktop are shown below:

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For more information on Citrix policy templates, please refer to Citrix eDocs - Manage Citrix Policy Templates.

A baseline policy configuration should also include Windows policies. Windows policies reflect user specific settings that optimize the user experience and remove features that are not required or desired in a XenDesktop environment. For example, one common feature turned off in these environments is Windows update. In virtualized environments, particularly where desktops and servers may be streamed and non-persistent, Windows update creates processing and network overhead, and changes made by the update process will not persist a restart of the virtual desktop or application server. Also in many cases, organizations use Windows software update service (WSUS) to control Windows updates. In these cases, updates are applied to the master disk and made available by the IT department on a scheduled basis.

In addition to the above considerations, an organization’s final baseline policy may include settings specifically created to address security requirements, common network conditions, or to manage user device or user profile requirements:

Printing

Citrix XenApp and Citrix XenDesktop support a variety of different printing solutions. In order to plan and successfully implement the proper printing solution it is important to understand the available technologies as well as their benefits and limitations.

Decision: Printer Provisioning

The process of creating printers at the start of a XenApp or XenDesktop session is called printer provisioning. There are multiple approaches available:

  • User Added - Allowing users to manually add printers gives them the flexibility to select printers by convenience. The drawback to manually adding network-based printers is that it requires the users to know the network name or path of the printers. There is also a chance that the native print driver is not installed in the operating system and the Citrix Universal Print Driver is not compatible, thereby requiring the user to seek administrative assistance. Manually adding printers is best suited in the following situations:
    • Users roam between different locations using the same client device (i.e. laptop, tablet).
    • Users work at assigned stations or areas whose printer assignments will rarely change.
    • Users have personal desktops with sufficient rights to install necessary printer drivers.
  • Auto Created - Auto-creation is a form of dynamic provisioning that attempts to create some or all of the available printers on the client device at the start of a user session. This includes locally attached printers as well as network-based printers. Auto-creating all client printers can increase the session logon time as each printer is enumerated during the logon process.
  • Session-Based - Session printers are a set of network-based printers assigned to users through a Citrix policy at the start of each session.
    • Proximity Based: Session printers filtred by IP subnet. The network printers created under this policy may vary based on where the user’s endpoint device is located. Proximity printing is recommended in situations where: Users roam between different locations using the same endpoint device (i.e. laptop, tablet) and where thin clients are used, which do not have the ability to connect to network-based printers directly.
    • Session printers can be assigned using the “Session Printer” policy or the “Printer Assignments” policy. The “Session printer” policy is intended to be used to set default printers for a farm, site, large group, or OU. The “Printer Assignments” policy is used to assign a large group of printers to multiple users. If both policies are enabled and configured, the session printers will be merged into a single list.
  • Universal Printer - The Citrix Universal Printer is a generic printer object that is auto-created at the start of a session and is not linked to a printing device. When using the Citrix Universal Printer it is not required to enumerate the available client printers during logon, which can greatly reduce resource usage and decrease user logon times. By default the Citrix Universal Printer will print to the client’s default printer, however the behavior can be modified to allow the user to select any of their compatible local or network-based printers.

    The Citrix Universal Printer is best suited for the following scenarios:
    • The user requires access to multiple printers both local and network-based which may vary with each session.
    • The user’s logon performance is a priority and the Citrix policy “Wait for printers to be created” must be enabled due to application compatibility.
    • The user is working from a Windows based device or thin client.

Note: Other options for provisioning printers, such as Active Directory group policy, “follow-me” centralized print queue solutions, and other 3rd party print management solutions can be used to provision printers into a Citrix session.

Decision: Printer Drivers

Managing print drivers in XenApp and XenDesktop can be a tedious task, especially in large environments with hundreds of printers. In XenApp and XenDesktop there are several methods available to assist with print driver management.

  • User Installed - When adding a printer within a XenApp or XenDesktop session and the native print driver is not available, the drivers can be installed manually, by the user. Many different print drivers can potentially be installed on different resources creating inconsistencies within the environment. Troubleshooting printing problems and maintenance of print drivers can become very challenging since every hosted resource may have different sets of print drivers installed. To ensure consistency and simplify support and troubleshooting, user installed drivers is not recommended.
  • Automatic Installation - When connecting a printer within a XenApp or XenDesktop session, a check is made to see if the required print driver is already installed in the operating system. If the print driver is not already installed, the native print driver, if one exists, will be installed automatically. If users roam between multiple endpoints and locations, this can create inconsistencies across sessions since users may access a different hosted resource every time they connect. When this type of scenario occurs, troubleshooting printing problems and maintenance of print drivers can become very challenging since every hosted resource may have different sets of print drivers installed. To ensure consistency and simplify support and troubleshooting, automatic installed drivers is not recommended.
  • Universal Print Driver - The Citrix Universal Printer Driver (UPD) is a device independent print driver, which has been designed to work with most printers. The Citrix Universal Printer Driver (UPD) simplifies administration by reducing the number of drivers required on the master image. For autocreated client printers, the driver records the output of the application and sends it, without any modification, to the end-point device. The endpoint uses local, device-specific drivers to finish printing the job to the printer. The UPD can be used in conjuction with the Citrix Universal Print Server (UPServer) to extend this functionality to network printers.

Decision: Printer Routing

Print jobs can be routed along different paths: through a client device or through a print server.

  • Client Device Routing - Client devices with locally attached printers (printers attached through USB, LPT, COM, TCP, etc.) will route print jobs directly from the client device to the printer.
  • Windows Print Server Routing - By default, print jobs sent to auto-created network-based printers will be routed from the user’s session to the print server. However, the print job will take a fallback route through the client device when any of the following conditions are true:
    • The session cannot contact the print server
    • The print server is on a different domain without a trust established
    • The native print driver is not available within the user’s session
  • Citrix Universal Print Server Routing - Print job routing follows the same process as Windows Print Server Routing except that the Universal Print Driver is used between the user’s session and the Citrix Universal Print Server.

The specifics with print job routing are based on the printer provisioning method. Auto-created and user-added printers can route print jobs based on the folloing diagrams:

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However, if the printers are provisioned as session printers, the print job routing options changes slightly. The jobs are no longer able to route through the user’s endpoint device.

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The recommended option is based on the network location of the endpoint device, the user’s session and the print server.

  • Client Device Routing
    • Use for locally attached printer implementations.
    • Use if a Windows endpoint device and printer are on the same high-speed, low-latency network as the Windows Print Server.
  • Windows Print Server Routing
    • Use if the printer is on the same high-speed, low-latency network as the Windows Print Server and user session.
  • Windows Print Server Routing (with Universal Print Server)
    • Use if non-Windows endpoint device and printer are on the same high-speed, low-latency network as the Windows Print Server.

Decision: Print Server Redundancy

Network-based printers, managed with a Microsoft print server or the Citrix Universal Print Server should be configured with redundancy in order to eliminate a single point of failure. The Citrix Universal Print Server should be defined within a Citrix Policy.

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Applications

Properly integrating an application requires understanding compatilibty and how the user/business requirements influences the appropriate delivery method.

Decision: Compatibility

VDI typically requires significant changes to be made to an organization’s application delivery and management strategy. For example, many organizations will take the opportunity to upgrade their desktop operating system and to simplify management by reducing the number of applications installed into the base image using techniques such as application streaming and application layering. These are significant changes that require comprehensive compatibility testing. Important compatibility requirements that may need to be verified include:

  • Operating system - the application must be compatible with the preferred operating system.
  • Multi-User -  Some applications may be more appropriate for delivery via a hosted shared desktop or a hosted Windows App. In these situations, the compatibility of the application must be verified against the multi-user capabilities of a server operating system like Windows Server 2012R2.
  • Application architecture - It is important to understand whether the application includes 16-bit, 32-bit or 64-bit code so that an appropriate operating system can be selected. 16-bit code cannot be executed on a 64-bit operating system. However, a 16-bit application can be delivered to users as a Hosted Windows App from a 32-bit desktop-based operating system like x86 editions of Windows 7, 8 or 10.
  • Interoperability - Some applications may experience complications if they coexist on the same operating system. Possible causes include shared registry hives, dll files or INI files as well as incompatible dependencies. Application interoperability issues should be identified so that appropriate remediation steps can be taken or an alternative delivery model selected.
  • Dependency - Applications may need to interact with each other to provide the users with a seamless experience. For example, applications that present information in a PDF format require a suitable PDF viewer to be available. Many times, the dependent (child) applications are version specific to the parent application.
  • Application virtualization - The use of application virtualization techniques, like streaming and layering, helps to simplify image management by reducing the number of applications installed into the base image. However, not all applications are suitable for streaming and layering because they may install device drivers, use COM+ or form part of the operating system.

Application compatibility can be achieved by doing a combination of manual, user testing, utilizing pre-verified lists maintained by the software vendor, or using an automated application compatibility solution, like Citrix AppDNA which runs through thousands of tests to verify compatibility.

Decision: Application Delivery Method

It is unlikely that a single delivery method will meet all requirements. Based on the outcome of the application categorization assessment process, several application delivery methods can be considered.

Choosing one of the appropriate application delivery method helps improve scalability, management and user experience.

  • Installed app - The application is part of the base desktop image. The install process involves dll, exe and other files being copied to the image drive as well as registry modifications.
  • Streamed App (Microsoft App-V) - The application is profiled and delivered to the desktops across the network on-demand. Application files and registry settings are placed in a container on the virtual desktop and are isolated from the base operating system and each other, which helps to address compatibility issues.
  • Hosted Windows App - The application is installed on a multi-user XenApp host and deployed as an application and not a desktop. The hosted Widnwos app is accessed seamless from the user’s VDI desktop or endpoint device, hiding the fact that the app is executing remotely.
  • Local App - The application is deployed on the endpoint device. The application interface appears within the user’s hosted VDI session even though it executes on the endpoint.

The following table provides recommendations on the preferred approaches for integrating applications into the overall solution:

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Virtual Machines

Virtual resources require proper allocation of the processor, memory and disk. These decisions have a direct impact on the the amount of hardware required as well as the user experience.

The key to successful resource allocation is to ensure that virtual desktops and applications offer similar levels of performance to physical desktops. Otherwise, productivity and overall user satisfaction will be affected. Allocating resources to the virtual machines above their requirements however is inefficient and expensive for the business.

The resources allocated should be based on the workload characteristic of each user group, identified during the assess phase.

Decision: Virtual Processor (vCPU)

For hosted desktop-based VDI models (hosted pooled desktops and hosted personal desktops), the general recommendation is two or more vCPUs per virtual machine so that multiple threads can be executed simultaneously. Although a single vCPU could be assigned for extremely light workloads, users are more likely to experience session hangs.

For hosted server-based VDI models (hosted Windows apps, hosted browser apps, hosted shared desktops), the proper vCPU allocation is based on the Non-Uniform Memory Access (NUMA) architecture of the processors.

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Each socket is divided into one or more NUMA nodes. Hosted server-based VDI models will often utilize 4 or more processors. Allocating more vCPU than the NUMA node contains results in a performance hit. Allocating a portion of a NUMA node to a virtual machine results in a perofmrance hit if the portion allocated is not easily divisible by the size of the NUMA node. It is often ideal to allocate the number of cores within a NUMA node to a virtual machine or allocate ½ of the cores to a virtual machine, while doubling the number of virtual machines.

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Note: Windows 2012R2 recommendations are based on the hosted Windows app, hosted browser app and hosted shared desktop VDI model.

Decision: Virtual Memory (vRAM)

The amount of memory allocated to each resource is a function of the user’s expected workload and application footprint. Assigning insufficient memory to the virtual machines will cause excessive paging to disk, resulting in a poor user experience; allocating too much RAM increases the overall cost of the solution.

The following table provides guidance on the virtual RAM that should be assigned based on workload:

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Note: Windows 2012R2 recommendations are based on the hosted Windows app, hosted browser app and hosted shared desktop VDI model.

Note: Memory allocation above 4GB requires a 64-bit operating system.

Note: If used, the Machine Creation Sevices and Provisioning Sevices cache in RAM amount should be added onto the virtual machine RAM specifications.

Decision: Disk Cache

The amount of storage that each VM requires will vary based on the workload and the image type. If creating hosted personal desktop without leveraging an image management solution, each VM will require enough storage for the entire OS and locally installed applications.

Deploying machines through Machine Creation Services or Provisioning Services can substantially reduce the storage requirements for each virtual machine. Disk space requirements for the write cache and difference disk will depend on application usage and user behavior. However, the following table provides a starting point for estimating disk space requirements based on machine sized with vCPU and vRAM as per the guidelines above:

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Decision: RAM Cache

Provisioning Services and Machine Creation Services have the capability to utilize a portion of the virtual machine’s RAM as a buffer for the storage cache. The RAM cache is used to improve the performance of traditional storage by sharing the virtual machine’s non-paged pool memory

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Note: If used, the Machine Creation Sevices and Provisioning Sevices cache in RAM amount should be added onto the virtual machine RAM specifications.

Note: If additional RAM is available on the host, the RAM Cache amounts can be increased to provide even greater levels of performance.

Decision: Storage IOPS

Storage performance is limited by the number of operations it can handle per second, referred to as IOPS. Underallocating storage IOPS results in a VDI desktop where apps, web pages and data are slow to load.

The following table provides guidance on the number of storage IOPS generated per user based on workload and operating system. Storage IO activity will be higher during user logon/logoff.

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Decision: Graphics (GPU)

Without a graphical processing unit (GPU), graphical processing is rendered with software by the CPU. A graphical processing unit (GPU) can be leveraged to improve server scalability and user experience or enable the use of graphically intensive applications. During the desktop design it is important to decide how the GPU (if used) will be mapped to the virtual machines. There are three methods available.

  • Pass-Through GPU - Each physical GPU is passed through to a single virtual machine (hosted apps or hosted desktops).
  • Hardware Virtualized GPU - Using a hypervisor’s vGPU technology, an NVIDIA GRID or Intel Iris Pro is virtualized and shared between multiple machines. Each virtual machine has the full functionality of GPU drivers and direct access to the GPU.
  • Software Virtualized GPU - The GPU is managed by the hypervisor and intercepts requests made by the VDI desktops. This process is used if a GPU is not installed within the host.
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User groups with a heavy use of graphical applications will often require the use of a NVidia hardware virtualized GPU. User groups who rely on office-based applications can have an observable benefit with the use of a hardware virtualized GPU from Intel.

Layer 4: The Control Layer

Active Directory

Decision: Forest Design

Multi-forest deployments, by default, do not have inter-domain trust relationships between the forests. An Active Directory administrator can establish trust relationships between the multiple forests, allowing the users and computers from one forest to authenticate and access resources in another forest.

For forests that have inter-domain trusts, Citrix recommends that the appropriate settings be configured to allow the Delivery Controllers to communicate with both domains. When the appropriate trusts are not configured, multiple XenDesktop sites for each forest must be configured. This section outlines the storage requirements on a per product basis and provides sizing calculations. For more information, please refer to Citrix article: CTX134971 - Successfully Deploying XenDesktop in a Complex Active Directory Environment.

Decision: Organizational Unit Structure

Infrastructure components for a XenApp and XenDestkop deployment should reside within their own dedicated organizational units (OUs); separating workers and controllers for management purposes. By having their own OUs, the objects inside will have greater flexibility with their management while allowing Citrix administrators to be granted delegated control.

A sample Citrix OU structure can be seen below.

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Decision: User Groups

Whenever possible, permissions and authorization should be assigned to user groups rather than individual users, thereby eliminating the need to edit a large amount of resource permissions and user rights when creating, modifying, or deleting user accounts.

Permission application example:

  • An application published to one group of 1,000 users requires the validation of only one object for all 1,000 users.
  • The same application published to 1,000 individual user accounts requires the validation of all 1,000 objects.

Database

The majority of Citrix products discussed within this document require a database. The following table outlines the usage on a per product basis:

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Decision: Edition

There are multiple editions of Microsoft SQL Server 2012: Express, Web, Standard, Business Intelligence, and Enterprise. Based on the capabilities of the various SQL Server editions available, the Standard edition is often used for hosting the XenApp and XenDesktop databases in production environments.

The Standard edition provides an adequate amount of features to meet the needs of most environments. For more information on the databases supported with Citrix products please refer to the Citrix Database Support Matrix. Different versions of Citrix products support different versions of the SQL server; therefore it is important to check the support matrix to ensure the version of SQL server used is compatible with the Citrix product being deployed.

Decision: Database Server Sizing

The SQL server must be sized correctly to ensure the performance and stability of an environment. Since every Citrix product uses SQL server in a different way, no generic all-encompassing sizing recommendations can be provided. Instead, per-product SQL server sizing recommendations are provided below.

XenApp and XenDesktop

XenApp and XenDesktop Brokers use the database as a message bus for broker communications, storing configuration data and storing monitoring and configuration log data. The databases are constantly in use and the performance impact on the SQL server can be considered as high.

Based on results from Citrix internal scalability testing the following SQL server specification for a server hosting all XenDesktop databases are recommended:

  • 2 Cores / 4 GB RAM for environments up to 5,000 users
  • 4 Cores / 8 GB RAM for environments up to 15,000 users
  • 8 Cores / 16 GB RAM for environments with 15,000+ users

The database files and transaction logs should be hosted on separate hard disk subsystems in order to cope with a high number of transactions. For example, registering 20,000 virtual desktops during a 15 minute boot storm causes ~500 transactions / second and 20,000 users logging on during a 30 minute logon storm causes ~800 transactions / second on the XenDesktop Site Database.

Provisioning Services

In addition to static configuration data provisioning servers store runtime and auditing information in the database. Depending on the boot and management pattern, the performance impact of the database can be considered as low to medium.

Based on this categorization, a SQL server specification of 4 Cores and 4 GB RAM is recommended as a good starting point. The SQL server should be carefully monitored during the testing and pilot phase in order to determine the optimal configuration of the SQL server.

Decision: Instance Sizing

When sizing a SQL database, two aspects are important:

  • Database file - Contains the data and objects such as tables, indexes, stored procedures and views stored in the database.
  • Transaction log file - Contains a record of all transactions and database modifications made by each transaction. The transaction log is a critical component of the database and, if there is a system failure, the transaction log might be required to bring the database back to a consistent state. The usage of the transaction log varies depending on which database recovery model is used:
    • Simple recovery - No log backups required. Log space is automatically reclaimed, to keep space requirements small, essentially eliminating the need to manage the transaction log space. Changes to the database since the most recent backup are unprotected. In the event of a disaster, those changes must be redone.
    • Full recovery - Requires log backups. No work is lost due to a lost or damaged database data file. Data of any arbitrary point in time can be recovered (for example, prior to application or user error). Full recovery is required for database mirroring.
    • Bulk-logged - Requires log backups. This is an adjunct of the full recovery model that permits high-performance bulk copy operations. It is typically not used for Citrix databases.

For further information, please refer to the Microsoft Developer Network – SQL Server Recovery Models.

In order to estimate storage requirements, it is important to understand the disk space consumption for common database entries. This section outlines the storage requirements on a per product basis and provides sizing calculations. For more information, please refer to Citrix article: CTX139508 – XenDesktop 7.x Database Sizing.

XenDesktop General

XenApp 7.x and XenDesktop 7.x use three distinct databases:

  • Site Configuration database - Contains static configuration and dynamic runtime data
  • Monitoring database - Contains monitoring data which is accessible through Director
  • Configuration logging database - Contains a record for each administrative change performed within the site (accessible through Studio)

Site Database

Since the database of a XenApp or XenDesktop site contains static configuration data and dynamic runtime data, the size of the database file depends not only on the physical size of the environment but also user patterns. The following factors all impact the size of the database file:

  • The number of connected sessions
  • The number of configured and registered VDAs
  • The number of transactions occurring during logon
  • The VDA heartbeat transactions

The size of the Site Database is based on the number of VDAs and active sessions. The following table shows the typical maximum database size Citrix observed when scale testing XenApp and XenDesktop with a sample number of users, applications, and desktop delivery methods.

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Note: This sizing information is a guide only. Actual database sizes may differ slightly by deployment due to how databases are maintained.

Determining the size of the transaction log for the Site database is difficult due to factors that can influence the log including:

  • The SQL Database recovery model
  • The launch rate at peak times
  • The number of desktops being delivered

During XenDesktop scalability testing, Citrix observed the transaction log growth rate at 3.5MB an hour when the system is idle, and a per user per day growth rate of ~32KB. In a large environment, transaction log usage requires careful management and a regular backup, to prevent excessive growth. This can be achieved by means of scheduled jobs or maintenance plans

Monitoring Database

Of the three databases, the Monitoring database is expected to be the largest since it contains historical information for the site. Its size is dependent on many factors including:

  • Number of Users
  • Number of sessions and connections
  • Number of workers
  • Retention period configuration – Platinum customers can keep data for over a year (default 90 days). Non-platinum customers can keep data for up to 7 days (default 7 days).
  • Number of transaction per second. Monitoring service tends to execute updates in batches. It is rare to have the number of transactions per second go above 20.
  • Background transaction caused by regular consolidation calls from the Monitoring service.
  • Overnight processing carried out to remove data outside the configured retention period.

The following table shows the estimated size of the Monitoring database over a period of time under different scenarios. This data is an estimate based on data seen within scale testing XenApp and XenDesktop (assuming a 5 day working week).

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Note: The 100,000 HSD tests are based on a test environment consisting of:

  •  2 Delivery Controllers
  • 43 Hosted Shared Desktop workers
  • 3 SQL servers, configured with databases held within one Always On Availability Group

For more information please see the Citrix Support article XenDesktop 7.x Database Sizing.

The size of the transaction log for the Monitoring Database is very hard to estimate, but XenApp and XenDesktop scalability testing showed a growth rate of about 30.5 MB an hour when the system is idle, and a per user per day growth rate of ~9 KB.

Configuration Logging Database

The Configuration Logging Database is typically the smallest of the three databases. Its size and the size of the related transaction log depends on the daily administrative activities initiated from Studio, Director or PowerShell scripts, therefore its size is difficult to estimate. The more configuration changes are performed, the larger the database will grow. Some factors that can affect the size of the database include:

  • The number of actions performed in Studio, Director and PowerShell.
  • Minimal transactions which occur on the database when no configuration changes are taking place.
  • The transaction rate during updates. Updates are batched whenever possible.
  • Data manually removed from the database. Data within the Configuration Logging Database is not subject to any retention policy, therefore it is not removed unless done so manually by an administrator.
  • Activities that have an impact on sessions or users, for example, session logoff and reset.
  • he mechanism used for deploying desktops.

In XenApp environments not using MCS, the database size tends to fall between 30 and 40MB. For MCS environments, database size can easily exceed 200MB due to the logging of all VM build data.

Temporary Database

In addition to the Site, Monitoring, and Configuration Logging databases, a system-wide temporary database (tempdb) is provided by SQL Server. This temporary database is used to store Read-Committed Snapshot Isolation data. XenApp 7.x and XenDesktop 7.x uses this SQL Server feature to reduce lock contention on the XenApp and XenDesktop databases. Citrix recommends that all XenApp 7.x and XenDesktop 7.x databases use Read-Committed Snapshot Isolation. For more information please see How to Enable Read-Committed Snapshot in XenDesktop.

The size of the tempdb database will depend on the number of active transactions, but in general it is not expected to grow more than a few MBs. The performance of the tempdb database does not impact the performance of XenApp and XenDesktop brokering, as any transactions that generate new data require tempdb space. XenApp and XenDesktop tend to have short-lived transactions, which help keep the size of the tempdb small.

The tempdb is also used when queries generate large intermediate result sets. Guidance and sizing the tempdb can be found on the Microsoft TechNet article Optimizing tempdb Performance.

Provisioning Services

The Provisioning Services farm database contains static configuration and configuration logging (audit trail) data. The record size requirements outlined below can be used to help size the database:

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During the PVS farm setup, a database with an initial file size of 20MB is created. Due to the nature of the data in the PVS farm database the transaction log is not expected to grow very quickly, unless a large amount of configuration is performed.

In contrast to XenApp, which also offers the ability to track administrative changes, the related information is not written to a dedicated database but directly to the Provisioning Services farm database. In order to limit the size of the Provisioning Services database it is recommended to archive the audit trail data on a regular schedule.

Decision: Database Location

By default, the Configuration Logging and Monitoring databases are located within the Site Configuration database. Citrix recommends changing the location of these secondary databases as soon as the configuration of the site has been completed, in order to simplify sizing, maintenance and monitoring. All three databases can be hosted on the same server or on different servers. An ideal configuration would be to host the Monitoring database on a different server from the Site Configuration and Configuration Logging databases since it records more data, changes occur more frequently and the data is not considered to be as critical as the other databases. For more information, please see Change secondary database locations in the Citrix Product Documentation.

Note: The location of the Configuration Logging database cannot be changed when mandatory logging is enabled.

Decision: High-Availability

The following table highlights the impact to XenApp, XenDesktop and Provisioning Services when there is a database outage:

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Note: Please review HA options for 3rd party databases (for example, App-V, SCVMM or vCenter) with the respective software vendor.

In addition to the built-in database redundancy options, Microsoft SQL Server, as well as the underlying hypervisor (in virtual environments), offer a number of high availability features. These enable administrators to ensure single server outages will have a minimal impact (if any) on the XenApp and XenDesktop infrastructure. The following the SQL / hypervisor high availability features are available:

VM-level HA - This high availability option is available for virtual SQL servers only, which need to be marked for High Availability at the hypervisor layer. In case of an unexpected shutdown of the virtual machine or the underlying hypervisor host, the hypervisor will try to restart the VM immediately on a different host. While VM-level HA can minimize downtimes in power-outage scenarios, it cannot protect from operating system level corruption. This solution is less expensive than mirroring or clustering because it uses a built-in hypervisor feature. However, the automatic failover process is slower, as it can take time detect an outage and start the virtual SQL server on another host. This may interrupt the service to users.

  • Mirroring - Database mirroring increases database availability with almost instantaneous failover. Database mirroring can be used to maintain a single standby or mirror database, for a corresponding principal or production database. Database mirroring runs with either synchronous operation in high-safety mode, or asynchronous operation in high- performance mode. In high-safety mode with automatic failover (recommended for XenDesktop) a third server instance, known as a witness, is required, which enables the mirror server to act as a hot standby server. Failover from the principal database to the mirror database happens automatically and is typically completed within a few seconds. It is a good practice to enable VM-level HA (or a similar automatic restart functionality) for at least the witness to ensure SQL service availability in case of a multi-server outage.

Note: Microsoft is planning to remove mirroring as a high availability option in a future release of SQL Server and is discouraging its use in new network development. Please refer to the Microsoft article Database Mirroring (SQL Server) for more information.

  • AlwaysOn Failover Cluster Instances - Failover clustering provides high-availability support for an entire instance of Microsoft SQL Server. A failover cluster is a combination of two or more nodes, or servers, using a shared storage. A Microsoft SQL Server AlwaysOn Failover Cluster Instance, introduced in SQL Server 2012, appears on the network as a single computer, but has functionality that provides failover from one node to another if the current node becomes unavailable. The transition from one node to the other node is seamless for the clients connected to the cluster. AlwaysOn Failover cluster Instances require a Windows Server Failover Clustering (WSFC) resource group. The number of nodes supported in the WSFC resource group will depend on the SQL Server edition. (Please refer to the table in the Decision: Edition earlier in this chapter.) For more information please refer to MSDN – AlwaysOn Failover Cluster Instances (SQL Server).
  • AlwaysOn Availability Groups - AlwaysOn Availability Groups is an enterprise-level high-availability and disaster recovery solution introduced in Microsoft SQL Server 2012, which enables administrators to maximize availability for one or more user databases. AlwaysOn Availability Groups require that the Microsoft SQL Server instances reside on Windows Server failover clustering (WSFC) nodes. Similar to failover clustering a single virtual IP / network name is exposed to the database users. In contrast to failover clustering, shared storage is not required since the data is transferred using a network connection. Both synchronous and asynchronous replication to one or more secondary servers is supported. As opposed to mirroring or clustering secondary servers can be actively used for processing incoming read-only requests, backups or integrity checks. This feature can be used to offload user resource enumeration requests to a secondary SQL server in XenDesktop environments to essentially scale-out a SQL server infrastructure. Since the data on active secondary servers can lag multiple seconds behind the primary server, the read-only routing feature cannot be used for other XenDesktop database requests at this point in time. For more information, please refer to MSDN – AlwaysOn Availability Groups (SQL Server).

The following table outlines the recommended high availability features for Citrix databases:

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Decision: Connection Leasing

Connection leasing is a new XenApp and XenDesktop 7.6 feature that allows Hosted Shared, Hosted Windows and Browser Apps and Personal VDI users to connect and reconnect to their most recently used applications and desktops, even when the site database is unavailable. Connection Leasing is not available for users with a Pooled VDI desktop.

The lease information along with the application, desktop, icon, and worker information is stored on the controller’s local disk and synchronized between controllers in the site. If the site database becomes unavailable, the controllers enter a “leased connection mode” and replay cached operations from an XML file on the local disk to connect or reconnect users to a recently used application or desktop.

Administrators familiar with the local host cache in XenApp 6.5 and earlier should understand the similarities and differences with connection leasing because it can have an impact on the design and scalability of the XenApp and XenDesktop 7.6 solution. In XenApp 6.5 and earlier, the IMA service is responsible for synchronizing the local host cache with the data store. In XenApp and XenDesktop 7.6, the FMA service caches the brokering operations (leases) to an XML file containing the address of the VDA, application path, and other details required for the session to launch. The FMA also caches dynamic information such as user sessions, VDA registrations, and load. These files are uploaded to the SQL database and synchronized between all controllers in the site. The controllers will download the files on a regular basis so that any other controller in the site can connect a user to their session.

Each controller needs additional disk space for the cached lease files. At a minimum, 4KB is required for each lease file. Each resource entry in the enumeration lease will take anywhere from 200 bytes to a few KBs depending on the number of entries and resources published. Citrix testing has shown that 200,000 leased connections for server hosted applications and desktops required approximately 3GB of disk space. 40,000 leased connections for assigned desktops required approximately 156MB of disk space.

By default, connection leases have an expiration period of two weeks. Applications and desktops must have been launched within the two last weeks to still be accessible when the database is unavailable. The expiration period is configurable using PowerShell cmdlets or editing the registry and can be set from 0 minutes to several years. Setting the expiration period too short will prevent users from connecting to their virtual desktops and applications in the event of an outage. Setting the expiration period too long will increase storage requirement on the controllers.

By default, connection leasing affects the entire site, however, leases can be revoked for specific users, which prevents them from accessing any applications or desktops when the site database is unavailable.

Citrix Licensing

Citrix offers organizations the flexibility of multiple licensing models that align with common usage scenarios. The different licensing models vary based on the Citrix product used, but can include per user/device and per concurrent user. Several Citrix products use the license server, while other products require a license to be installed on the product itself.

For more information on XenDesktop 7.x licensing, refer to CTX128013 - XenDesktop Licensing.

For more information on Microsoft Licensing, refer to the Microsoft document – Licensing Microsoft’s Virtual Desktop Infrastructure Technology.

Decision: Sizing

Internal scalability testing has shown that a single virtual license server with two cores and 2GB of RAM can issue approximately 170 licenses per second or 306,000 licenses per half hour. If necessary, the specification of the license server can be scaled out to support a higher number of license requests per second.

Decision: High Availability

For a typical environment, a single license server is sufficient. Should the license server become unavailable, dependent Citrix products will enter a 30-day grace period, which provides more than enough time to resolve connectivity issues and/or restore or rebuild the license server.

Note: If the license server and the Citrix product do not communicate within 2 heartbeats (5-10 min), the Citrix product will enter a grace period and will allow connections for up to 30 days. Once communication with the license server is re-established, the license server will reconcile the temporary and actual licenses.

Note: A CNAME record in DNS is a convenient way to reference the license server. Using CNAMEs allows the license server name to be changed without updating the Citrix products.

If additional redundancy is required, Citrix supports the following high availability solutions for the license server.

  • Windows Clustering – Cluster servers are groups of computers that work together in order to increase availability. Clustering allows the license server role to automatically failover in the event of a failure. For more information on clustering, please see the Citrix eDocs article – Clustered License Servers.
  • Duplication of license server – Create a VM level backup of the license server. This backup should not be stored on the same host as the license server. Instead, it should be stored in a safe location, such as a highly available storage solution, or backed up to tape or disk. The duplicate server is not active, and will remain on standby until the need arises to restore the active license server. Should the license server be restored using this backup, any new licenses must be re-downloaded to the server. 

Each method allows an administrator to exchange a single license server for another without an interruption in service; assuming that the change occurs during the grace period and that the following limitations are considered.

  • License files will reference the server specified during the allocation process. This means that the license files can only be used on a server with the same binding information (Hostname) as the server that was previously specified.
  • Two Windows-based, domain joined license servers cannot share the same name and be active in the environment at the same time.
  • Because license servers do not communicate with each other, any additional licenses must be placed on both the active and backup license server.

Decision: Optimization

License server performance can be optimized by tuning the number of “receive” and “processing” threads. If the thread count is set too low, requests will be queued until a thread becomes available. Conversely, if the thread count is set too high, the license server will become overloaded. The optimal values are dependent on the server hardware, site configuration, and license request volume. Citrix recommends testing and evaluating different values to determine the proper configuration. Setting the maximum number of processing threads to 30 and the maximum number of receiving threads to 15 is a good starting point for large scale deployments. This optimization will improve the Citrix License Server ‘s ability to provide licenses by increasing its ability to receive and process license requests.