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Another Citrix issue where everything initially looks correct.
You update the environment to VDA 2607.
MCS provisions the machine.
The write-cache disk is present and correctly attached to the VM.
VDA registered.
Users can work.
Looks good.
Except for one small detail:
👉 The MCSIO write-cache disk stays empty.
The combination matters
Citrix currently documents this issue under PMCS-63379 for this exact combination:
**Windows Server 2019 * VDA 2607 * MCSIO**
The cache disk is successfully attached to the provisioned VM, but is not used as a write-back cache as intended.
The tricky part:
If your upgrade checks only confirm that the cache disk exists, this issue is easy to overlook.
The workaround is almost typically Citrix 😎
On the master image, open the following registry path:
|
1 2 |
HKLM\SYSTEM\CurrentControlSet\Services\CVhdFilter |
Create an empty registry key named:
Parameters
Yes.
An empty registry key.
Then:
Shut down/reseal the master image → create a new snapshot → update the MCS catalog → restart the machines.
Afterwards, normal user activity should populate the write-back cache disk again.
Why I find this interesting
MCSIO exists specifically to absorb writes through RAM and the temporary cache disk.
Citrix uses the following file as part of this:
mcsdif.vhdx
If the disk is attached but not used, simply looking at the VM configuration is not enough.
My recommendation after a VDA upgrade
With MCSIO, do not check only:
✅ Cache disk attached
Also check:
✅ MCSIO driver present
✅ Cache actually active
✅ mcsdif.vhdx present
✅ Write-cache disk receives writes
✅ Catalog update works
Because:
Disk attached apparently does not automatically mean cache working. 😉
Affected: VDA 2607 + Windows Server 2019 + MCSIO
Citrix issue: PMCS-63379
