Manage virtual machines
This section provides an overview of how to create Virtual Machines (VMs) using templates. It also explains other preparation methods, including cloning templates and importing previously exported VMs.
What is a virtual machine?
A Virtual Machine (VM) is a software computer that, like a physical computer, runs an operating system and applications. The VM comprises a set of specification and configuration files backed by the physical resources of a host. Every VM has virtual devices that provide the same functions as physical hardware. VMs can give the benefits of being more portable, more manageable, and more secure. In addition, you can tailor the boot behavior of each VM to your specific requirements. For more information, see VM Boot Behavior.
Citrix Hypervisor supports guests with any combination of IPv4 or IPv6 configured addresses.
In Citrix Hypervisor VMs can operate in full virtualized (HVM) mode. Specific processor features are used to ‘trap’ privileged instructions that the virtual machine carries out. This capability enables you to use an unmodified operating system. For network and storage access, emulated devices are presented to the virtual machine. Alternatively, PV drivers can be used for performance and reliability reasons.
VMs are prepared from templates. A template is a gold image that contains all the various configuration settings to create an instance of a specific VM. Citrix Hypervisor ships with a base set of templates, which are raw VMs, on which you can install an operating system. Different operating systems require different settings to run at their best. Citrix Hypervisor templates are tuned to maximize operating system performance.
There are two basic methods by which you can create VMs from templates:
Using a complete pre-configured template, for example the Demo Linux Virtual Appliance.
Installing an operating system from a CD, ISO image or network repository onto the appropriate provided template.
Windows VMs describes how to install Windows operating systems onto VMs.
Linux VMs describes how to install Linux operating systems onto VMs.
Templates created by older versions of Citrix Hypervisor can be used in newer versions of Citrix Hypervisor. However, templates created in newer versions of Citrix Hypervisor are not compatible with older versions of Citrix Hypervisor. If you created a VM template by using Citrix Hypervisor 8.2, to use it with an earlier version, export the VDIs separately and create the VM again.
In addition to creating VMs from the provided templates, you can use the following methods to create VMs.
You can make a copy of an existing VM by cloning from a template. Templates are ordinary VMs which are intended to be used as master copies to create instances of VMs from. A VM can be customized and converted into a template. Ensure that you follow the appropriate preparation procedure for the VM. For more information, see Preparing for Cloning a Windows VM Using Sysprep and Preparing to Clone a Linux VM.
Templates cannot be used as normal VMs.
Citrix Hypervisor has two mechanisms for cloning VMs:
A full copy
The faster Copy-on-Write mode only writes modified blocks to disk. Copy-on-Write is designed to save disk space and allow fast clones, but slightly slows down normal disk performance. A template can be fast-cloned multiple times without slowdown.
If you clone a template into a VM and then convert the clone into a template, disk performance can decrease. The amount of decrease has a linear relationship to the number of times this process has happened. In this event, the
vm-copyCLI command can be used to perform a full copy of the disks and restore expected levels of disk performance.
If you create a template from VM virtual disks on a shared SR, the template cloning operation is forwarded to any server in the pool that can access the shared SRs. However, if you create the template from a VM virtual disk that only has a local SR, the template clone operation is only able to run on the server that can access that SR.
You can create a VM by importing an existing exported VM. Like cloning, exporting and importing a VM is fast way to create more VMs of a certain configuration. Using this method enables you to increase the speed of your deployment. You might, for example, have a special-purpose server configuration that you use many times. After you set up a VM as required, export it and import it later to create another copy of your specially configured VM. You can also use export and import to move a VM to the Citrix Hypervisor server that is in another resource pool.
For details and procedures on importing and exporting VMs, see Importing and Exporting VMs.
Citrix VM Tools
Citrix VM Tools provide high performance I/O services without the overhead of traditional device emulation.
Citrix VM Tools for Windows consist of I/O drivers (also known as paravirtualized drivers or PV drivers) and the Management Agent.
The I/O drivers contain storage and network drivers, and low-level management interfaces. These drivers replace the emulated devices and provide high-speed transport between Windows and the Citrix Hypervisor product family software. While installing a Windows operating system, Citrix Hypervisor uses traditional device emulation to present a standard IDE controller and a standard network card to the VM. This emulation allows the Windows installation to use built-in drivers, but with reduced performance due to the overhead inherent in emulating the controller drivers.
The Management Agent, also known as the Guest Agent, is responsible for high-level virtual machine management features and provides a full set of functions to XenCenter.
Install Citrix VM Tools for Windows on each Windows VM for that VM to have a fully supported configuration, and to be able to use the xe CLI or XenCenter. A VM functions without the Citrix VM Tools for Windows, but performance is hampered when the I/O drivers (PV drivers) are not installed. You must install Citrix VM Tools for Windows on Windows VMs to be able to perform the following operations:
Cleanly shut down, reboot, or suspend a VM
View VM performance data in XenCenter
Migrate a running VM (using live migration or storage live migration)
Create snapshots with memory (checkpoints) or revert to snapshots
For more information, see Install Citrix VM Tools for Windows.
Citrix VM Tools for Linux contain a guest agent that provides extra information about the VM to the host.
You must install the Citrix VM Tools for Linux on Linux VMs to be able to perform the following operations:
View VM performance data in XenCenter
Adjust the number of vCPUs on a running Linux VM
Enable dynamic memory control
For more information, see Install Citrix VM Tools for Linux.
XenCenter reports the virtualization state of a VM on the VM’s General tab. You can find out whether or not Citrix VM Tools are installed. This tab also displays whether the VM can install and receive updates from Windows Update. The following section lists the messages displayed in XenCenter:
I/O optimized (not optimized): This field displays whether or not the I/O drivers are installed on the VM.
Management Agent installed (not installed): This field displays whether or not the Management Agent is installed on the VM.
Able to (Not able to) receive updates from Windows Update: specifies whether the VM can receive I/O drivers from Windows Update.
Windows Server Core 2016 does not support using Windows Update to install or update the I/O drivers. Instead use the Citrix VM Tools for Windows installer provided on the Citrix Hypervisor downloads page.
Install I/O drivers and Management Agent: this message is displayed when the VM does not have the I/O drivers or the Management Agent installed.
Supported guests and allocating resources
For a list of supported guest operating systems, see Supported Guests, Virtual Memory, and Disk Size Limits
This section describes the differences in virtual device support for the members of the Citrix Hypervisor product family.
The current version of the Citrix Hypervisor product family has some general limitations on virtual devices for VMs. Specific guest operating systems may have lower limits for certain features. The individual guest installation section notes the limitations. For detailed information on configuration limits, see Configuration Limits.
Factors such as hardware and environment can affect the limitations. For information about supported hardware, see the Citrix Hypervisor Hardware Compatibility List.
Citrix Hypervisor emulates an IDE bus in the form of an
hd* device. When using Windows, installing the Citrix VM Tools installs a special I/O driver that works in a similar way to Linux, except in a fully virtualized environment.