Extend Persistent Vdisk ?

Write cache will be discarded any time you modify the OS Image, for example if you perform OS Image patching.
Hence it is best practice to store user data and settings seperately from the device using technologies like folder redirection, roaming profiles etc. to not loose these kind of things when you switch to a new OS Image (i.e. Win7 to Win8).
Because you cannot extend the vDisk while it is in use, the process requires you to create a copy and modify this one while it is not registered in the WSM Admin Console.
Is the OS Image you are trying to extend single partition or multi partition? Extending a multi-partition OS Image is not supported in the current version, but you could use reverse image capture featur as it would combine the separate partitions in the vdisk into a single partition on the client. You can then re-capture the OS image with larger partition sizes.

When you start the patch process you can select the partitions to update and the write cache files for these partition will be discarded.
The partitions which are not patched and remain using Persistant Mode will keep their write cache files.
So if you select to only patch the OS partition, the write cache for the user partition will remain intact.

Please bear in mind that whenever you assign a new/different OS Image to the device, this is like a fresh install of Windows and all User settings and data will not be available to a user working on this image as the write cache always belongs to the OS Image it is created for this machine. Hence my advice to consider folder redirections, roaming profiles etc. to seperate these data and settings from the Windows installation on the device, which in WSM world a compbination of the OS Image pluse the per-machine, per-partition write cache file.

Thank you for your help
I asked if I make a new persistent image with 2 partitions , one for OS and another one for the user profiles,
What about the write cache when we make a patch it will be deleted or no?
We don't have a roaming profile or folder redirection, what is your recommendation to keep the write cache when we patching the image?
Thank you