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Take care – Express Patch 6 for ESXi 6 can break your Backup (CBT Bug)!

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Just a short notice if you are already on ESXi 6:

There is an issue with Change Block Tracking (CBT) in Express Patch 6 for ESXi 6 (Build 3825889) that can affect your backup if your backup software relies on CBT.

  • affected are only VMs with Operating System Server 2008 or newer (Linux VMs are unaffected.)
  • all vHW Versions (7, 8, 9, 10 and 11) and all VMware tools versions are affected
  • the issue occurs with thick and thin provisioned disks
  • the issue only occurs when the backup software uses quiesced snapshots

Error messages:

You can find errors in the vmware.log of the affected VMs similar to:

vcpu-0| I120: SNAPSHOT:SnapshotBranchDisk: Failed to acquire current epoch for disk /vmfs/volumes/
vmdk : Change tracking is not active for this disk 572.

Other symptoms:

  • you have to backup an abnormal amount of data (it is almost a full backup even if it is an incremental one)
  • longer backup durations due to the abnormal amount of data sent to the backup software

Resolution:

 

There are currently 2 workarounds available:

  1. don’t use quiesced (application-aware) snapshots
  2. Revert to a previous version of ESXi, e.g. vSphere ESXi 6 U2 (build 3620759) (https://kb.vmware.com/kb/1033604)

If you already have affected virtual machines, migrate them to a host using an unaffected ESXi version (eg. VMware ESXi 6 Update 2, build 3620759).

Then you have to reset CBT of the affected VMs and perform a Full Backup to ensure that your backup is reliable.

 

Additional information:

  • VMware VDP seems to be affected by the issue
  • IBM TSM seems to be affected by the issue
  • Veeam Backup & Replication seems not to be affected as long as you do not enable VMware Tools quiescence (in the backup job settings):

enable_guest_quiescence

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