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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

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