KB2913695–OffloadWrite Does PrepareForCriticalIo For Whole VHD On WS2012 Or WS2012 R2 Hyper-V

Microsoft has published a hotfix for when OffloadWrite is doing PrepareForCriticalIo for the whole VHD in a Windows Server 2012 or Windows Server 2012 R2 Hyper-V.


Consider the following scenario:

  • You have a Hyper-V host that is running Windows Server 2012 or Windows Server 2012 R2.
  • You run a copy for a file in a virtual machine.
  • There is an offload write for Virtual Hard Disk (VHD) in the host.

In this scenario, NTFS in the host would do PrepareForCriticalIo for the whole VHD. This operation may cause the following bad consequences for Cluster Shared Volumes:

  • Redirected I/O may be time-out.
  • Snapshot creation can be stuck until the offloadWrite is complete.
  • Volume dismount will be blocked by inflight I/O. This can cause the Physical Disk Resource to be detected as deadlocked if dismount takes more than 3 minutes, or the cluster node to be bug checked if dismount takes more than 20 minutes.

A hotfix is available for this issue.

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