Product Documentation

Configure vDisk caching

Aug 30, 2017

vDisk caching enables a PVS proxy to reside in Dom0 (XenServer’s Control Domain) on a XenServer host where streaming of a PVS vDisk is cached at the proxy before being forwarded to the VM. Using the cache, subsequent booting (or any IO requests) of the VM on the same host can be streamed from the proxy rather than streaming from the server over the network. Using this model, more local resources on the XenServer host are consumed, but streaming from the server over the network saves resources, effectively improving performance.

With the proxy feature:

  • PVS and XenServer provide an improved functional paradigm by providing a unique value available when used together.
  • PVS provides support for local, NAS and SAN attached storage in XenServer.
  • Environments experience reduced network traffic.
  • Deployments experience improved fault tolerance, with tolerance for outage instances of a PVS server.

Important

The proxy feature is only supported on XenServer with the proxy capability installed. UI changes only occur when you are using that type of hypervisor. To use this feature, an optional package must be installed on the XenServer host(s). There are no additional dependencies on the installer.

For more information on the relationship between XenServer and PVS, refer to the blog XenServer and PVS: Better Together.

Citrix recommends that you do not disable the proxy feature on a VM using the XenServer console. When disabled using this method, PVS fails to recognize the configuration change and continues to believe that the proxy feature is enabled on that VM. If you want to disable the proxy feature for a single device, issue the following PowerShell command:

Set-PvsDevice -Name <deviceName> -EnableXsProxy 0

Using the XenServer Proxy Feature

The proxy feature is only supported on XenServer with the proxy capability installed. UI changes only occur when you are using that type of hypervisor. To use this feature, an optional package must be installed on the XenServer host(s). There are no additional dependencies on the installer.

Before using this feature the XenServer administrator must create a PVS Site object using the XenServer console. This effectively configures the storage (i.e., storage repositories) that will be used when proxying the IO requests. This work must be performed on XenServer.

Consider the following when using the proxy feature with XenServer:

  • A XenServer PVS Site object must be created and configured with the storage repository (SR) before the PVS Console can proxy on the VM.
  • PVS calls the XenServer API to check if the proxy feature is enabled before it exposes any PVS/XenServer proxy interfaces. 
  • PVS configures the XenServer proxy for devices using the XenDesktop Setup Wizard and the VM Setup Wizard. 
  • PVS targets are aware of the their proxy status; once the feature is installed, no additional configuration tasks are required.

In environments where two PVS servers reside with the same VHD but have different filesystem timestamps, the data is cached twice. Due to this limitation, Citrix recommends that you use VHDX rather than VHD.

Configuring the XenServer Proxy feature

Use the XenDesktop Setup Wizard and the Streaming Wizard to access the proxy feature.  Both Wizards are similar, and share many of the same screens. The following differences exist:

  • The XenDesktop Setup Wizard is used to configure VMs running on a hypervisor (for example, XenServer, Esx, or HyperV/SCVMM) that is controlled using XenDesktop.
  • The Streaming Wizard is used to create VMs on a XenServer host; it does not involve XenDesktop.

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The proxy feature is only supported on XenServer that has the capability installed. UI changes captured in this section only apply when you are using that type of hypervisor.

When a proxy cache configuration is tied to a PVS server, and you reinstall XenServer on the host that had the proxy cache configuration, PVS and XenServer become out of sync. This occurs because the reinstallation of XenServer wipes the previously configured proxy cache configuration.

In this scenario, PVS assumes that the proxy cache configuration still exists, and when the Streamed VM Setup Wizard is used, it fails, indicating that the provided UUID (associated with the proxy configuration) is invalid. For this reason, the user must delete all previously configured VMs associated with this cache configuration, including the host. After accomplishing this, reconfigure PVS and setup the cache again.

To configure the proxy feature, select one of the Wizards based on how you intend to use it (XenDesktop Setup Wizard or Streamed VM Setup Wizard) in the PVS Console:

  1. Navigate to a site.
  2. Select the site, then right click to expose a contextual menu.
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3. Select the appropriate Wizard based on how you intend to use the proxy feature.

Using Wizards to configure vDisk caching

To use vDisk caching, first determine how you’ll use it. If you are:

  • configuring VMs running on a hypervisor controlled by XenDesktop, use the XenDesktop Setup Wizard.
  • creating VMs on a XenServer host that does not involve XenDesktop, use the Streamed VM Setup Wizard.

Configure vDisk caching using the Streamed Virtual Machine Setup Wizard

The Streamed Virtual Machine Setup Wizard was modified to include a new checkbox to enable the vDisk caching feature.  After invoking the Wizard, select Enable XenServer vDisk caching for all Virtual Machines:

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After selecting Enable XenServer vDisk caching for all Virtual Machines, all VMs that are created using the Wizard are configured to use the proxy (vDisk caching) feature.

After selecting Enable XenServer vDisk caching for all Virtual Machines, the following screen appears (the first time vDisk caching is enabled for the host) after clicking Next:

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The Wizard allows you to select the XenServer PVS Site to which you want to apply vDisk caching.  In the XenServer screen, a drop down list displays the list of all the PVS Site objects on XenServer that have been configured but not yet associated with a PVS site.

In the drop down menu, select a PVS Site to associate vDisk caching. After selecting it, the site is now associated with the PVS site that was selected from which to run the Wizard.

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The next time this Wizard is run for the same PVS site using the same XenServer, this page is not displayed.