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Tuesday, 28 October 2014

VMware, Changed Block Tracking, disk expansion and silent backup corruption

VMware have released this Knowledge Base article.

If you have Change Block Tracking turned on (which is a sensible thing to do) and then you expand a virtual disk to be larger than 128GB, the data provided to the backup provider is wrong.

I haven't verified this with Data Protector, but I can't see any reason why DP wouldn't be affected in the same way as (say) Veeam, which uses the same mechanism.

Solution: turn off change block tracking, run a backup, and then turn change block tracking on again.

Greg Baker is an independent consultant who happens to do a lot of work on HP DataProtector. He is the author of the only published book on HP Data Protector ( He works with HP and HP partner companies to solve the hardest big-data problems (especially around backup). See more at IFOST's DataProtector pages at