Next, using specific PVS client software, which will need to be installed on Master Target Devices as well. vDisks can be stored locally or on a shared storage accessible by all provisioning servers. vDisks can be assigned to multiple target devices in read-only mode, also referred to as Standard Image mode, meaning that the vdisk will be shared by multiple physical and/virtual devices at the same time. Each Machine will have a Write Cache location assigned to it where all the writes to the (read-only) vdisk will be stored.
However, vDisks can also be assigned in a one-to-one fashion, which is referred to as Private Image Mode, allowing the user to read and write to the vdisk. All changes made will be saved. To update any application/OS in master image(vdisk), it need to be kept under Private Image mode.
Vdisk’s life cycle, versioning
The life cycle of a vDisk is pretty basic, at least in theory . After creating a vDisk and assigning to multiple target devices, which is basically step one, it will need to be maintained and updated from time to time. Finally , when no longer in user , a vDisk might need to be retired and taken out of production.
Provisioning Services Versioning technology used for updating and maintenance purposes
When updating and maintaining a vDisk (in read-only standard mode) when using PVS versioning, it will involve creating a new version of current vDISK,also known as Differencing disk, which will be then linked to the original vDisk.
When versioning is used, this process can be automated, but it can be done manually as well. Next, the newly provisioned (differencing) vDisk needs to be assigned to the Master Target device mentioned earlier and booted in maintenance mode, something which is also easily done from the versioning console. Once booted, you are free to make your changes and shutdown the Master Target Device after you are done.
Note: vDisk updates can be automated and scheduled. This feature supports updates detected and delivered from WSUS and SCCM.
As a final step, the updated vDisk will need to be promoted to either Test or Production. This step can also be automated and scheduled when needed.
Beware that, when you create multiple ‘new’ versions, these will be differencing disks pointing back to the original vDisk. Citrix advises merging the differencing disks back into the base image whenever you have created 3 to a maximum of 5 differencing disks a.k.a as a chain of differencing disks. This will not only save you some disk space, but will also positively impact performance.
Note: Be aware that while promoting the version, PVS will actually open up the vDisk and write to it. This it can lead to inconsistencies if you are storing vDisks locally and replication can be complicated.
Creating Versions from Provisioning console
The first version, when creating the vDisk, is called Base (version 0). At file level, the Base consists of a .vhd file (the vDisk itself), a .pvp file (the vDisk properties file) and the .lok file (when the vDisk is in use).