New issue was discovered which may result in corrupted backups leveraging vSphere CBT (Change Block Tracking) for incremental backups. Basically all backup products which use VMware API for Backups ( Virtual Disk Development Kit (VDDK) )are affected.
This issue occurs when you expand a virtual disk vmdk file which has Change Block Tracking (CBT) enabled past any 128GB boundary (see Additional Information section). 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, so that a restoring from an incomplete backup could cause a data loss.
Issue is affecting VMware ESXi 4.x and ESXi 5.x.
Backup can be inconsistent after disk extension passing any of these boundaries:
If initial disk size was 300GB and you are extending to 500GB you are not affected, but from 300GB to 520GB then yes and so on…
There is no fix available for now, just workaround published by VMware:
After having extended the CBT-enabled disks past a 128GB boundary:
- Turn off CBT
- Take a snapshot (Does suspend/resume)
- Delete the snapshot (To recover space and performance)
- Turn on CBT
- Take a snapshot (Again with the suspend/resume)
- Delete the snapshot
The next backup after toggling CBT is the full backup of the virtual machine.
Note: Discard any backups that were captured after growing the disk, as they can be incomplete.
UPDATE:Veeam already released fix for this issue for B&R v7 and B&R v8 has built-in fix, you can find more info including their script in KB1940
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