VMware releases bugfix for change block tracking (CBT) data corruption

VMware released on January 27 an important patch for vSphere 5.0, 5.1 and 5.5. This fixes a bug in Change Block Tracking which could lead to customers not being able to restore virtual machines due to missing blocks!

Make sure to apply this patch if you are using CBT and expanded the size of virtual disks!

This issue occurs when you expand a virtual disk .vmdk file which has Change Block Tracking (CBT) enabled past any 128 GB boundary. When the disk is extended the change tracking data becomes unreliable. Due to the faulty changed block information, some changed blocks might not be captured by a backup. Consequently, a restore operation that uses the incomplete backup could cause a data loss. More information here.

The issue is described by Symantec here. A lot of discussion about this issue in the Veeam forum.

Veeam has a KB article about the CBT corruption.

The patch is VMware ESXi 5.5, Patch ESXi-5.5.0-20150104001-standard (2099271) and more information can be found here.

The description of the fix is:

When you use backup software that uses the Virtual Disk Development Kit (VDDK) API call QueryChangedDiskAreas(), the list of allocated disk sectors returned might be incorrect and incremental backups might appear to be corrupt or missing. A message similar to the following is written to vmware.log:

DISKLIB-CTK: Resized change tracking block size from XXX to YYY

For more information, see KB 2090639.


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