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VM RAM is consuming Write Cache on Provisioned VMs


I'm seeing a new (to me) behavior. My PVS Target devices are thick provisioned with 30GB cache disks. VM Objects have 12 GB of ram. When the VMs boot, about 12GB of write cache appears to be unavailable, consumed by the amount of RAM the VM is set to. If a VM is shut down and the amount of RAM is changed, the size of the cache disk changes to reflect the new size of the RAM (If I drop the VM to having 8GB of ram, about 8GB of cache is unavailable.) Win10 and 11 vdisks are set to standard image, cache in device ram with overflow on hard disk, 1024MB.

I just built new images for Windows 11 desktops and Server 2022 App Servers, hypervisor is vmware, PVS is version 2203 LTSR CU3 (2203.0.17).  Our Win10 VMs that have been around for years do not exhibit this behavior.

My page file is set like the Win10 boxes I'm trying to get rid of. The screen shot shows 12GB or ram, and the roughly 30GB that should be cache disk being reduced to 17GB. 


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Opened a support case but it was not successful. Support assumed write cache was "filling up" and this was not the case.

Write Cache was "MISSING".  WCDisk was 30GB but PVS Virtual Disk Status says the Cache was 17GB. Missing amount matched the amount of RAM assigned to the VM.
Issue has been resolved by doing the following:
Reverted disk to private image. 
Re-ran PVS Optimization tool. 
Converted back to standard image with cache in RAM with overflow on disk. 

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If you choose the Ram cache to be larger than the VMs available ram you are going to have huge paging problems.  You don't set any settings for how much disk cache you will use, so your writecache drive should be the size of your provisioned C:\ drive, and thin provisioned only.  There is no need for thick provisioning of the storage, you don't get any performance benefit.

If your VM has 12GB of ram, I would only set the Ram cache to 1GB.  To do 10GB of cache you will need 48GB of ram or more on the VM.  I did create a little script that allows you to query the VM to see exactly how much RAM cache you are using versus how much disk cache.



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