Great blogpost by Robin Hobo website explaining how to create an XenApp 6.5 vDISK
After installing and configuring Citrix Provisioning Services Server (as described here) and the Citrix XenApp 6.5 Controller (as described here) it’s time to create the Citrix XenApp vDisk, also known as the “Golden disk”, for the Citrix XenApp 6.5 running in Session-host mode only.
It is highly recommended to use Citrix Provisioning Services when deploying multiple XenApp servers in your farm (hardware must be equal). With Citrix Provisioning Services you can provide a single vDisk for all your XenApp servers. Booting all the XenApp servers from the same vDisk ensures that they all are equal, and since the vDisk is read only, all servers will be in “original clean state” after a reboot.