2014
03.06

Microsoft released a hotfix for WS2012 and WS2012 R2 to deal with a scenario where CSV failover time is longer than expected in Windows failover cluster.

Symptoms

In a Windows failover-cluster that uses Cluster Shared Volumes (CSV), the diff area that is allocated by Volsnap is large and fragmented. In this situation, you encounter the following issues:

  • The failover time on the CSV is longer than expected.
  • The time that Volsnap takes to mount or unmount snapshots is several minutes.

More Information

When a NTFS or ReFS volume is mounted or dismounted, Volsnap iterates through the diff area to mount or unmount the snapshots that belong to that volume. When the diff area allocation becomes large and fragmented, the time that Volsnap takes to mount or unmount operations could be several minutes. Additionally, failover time can be longer than expected.

The resolution is … hmm … long. It is related to two updates:

Two new cluster Physical Disk resource private properties were added, and they can be manipulated to resolve the issue:

  • SnapshotDiffSize: This property controls the maximum diff area size that can be consumed by Volsnap for a Physical Disk resource configured for CSV. Units: In MB (DWORD), Default Value: 0, Maximum Value: 1 TB, The Physical Disk resource must be taken offline/online for changes to take effect.
  • SnapshotAgeLimit: This property is aResource Type private property of the Physical Disk to control the maximum age of a snapshot. Long lived snapshots are a significant contributor to diff area fragmentation. Units: In Days (DWORD), Default Value: 7, Range: 1-60 , This is a global property which affects  all Physical Disk resources. You do not have to take the resource offline or online for it to take effect.

Get-ClusterSharedVolume <Cluster Disk Name> | Set-ClusterParameter snapshotdiffsize <Snapshot Diff Size in MB>

Get-ClusterResourceType "physical disk" | Set-ClusterParameter snapshotagelimit <Snapshot Age in Days>

My advice: leave well alone and only manipulate these settings under the advice of Microsoft support (not some local dude, but actual Premier support).

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