Bitmap and delta sync

  • What’s the definition of “bitmap sync” and “delta sync”? What’s the differences/similarities between these 2?

    George, longer definitions are in the online help for Zerto and also in the admin guides, but the short version:

    • Bitmap Sync – when Zerto sites are disconnected or the ability to replicate is otherwise severely impacted, Zerto can track changes in a bitmap of pointers\references so that when the issue is resolved it can quickly bundle up those blocks that changed during the outage and ship them over to the relevant target journals
    • Delta Sync – when Zerto needs to verify disk integrity, for example in the event of a failover with Reverse Protection enabled, it will go through a checksum validation between the source and target disks and send any relevant missing writes from the source to the target

    Thanks Sean. I tried looking in the online help and Admin guide, which both referred to bitmap and delta syncs, but didn’t actually define what they were. But thanks again.


    Apologies for not being more clear and pointing the pages out. A long-form definition of Bitmap Sync can be found in the latest Zerto Virtual Manager vSphere Administration Guide which I’ve attached to this comment. Starting on page 158 under “Bitmap Syncing”, with Delta on the following page.

    Hope this helps!

    You should also be aware, that depending on the duration of the outage, what would of been a bitmap sync, can turn into a delta sync. If the outage is long enough, and the data change rate is high enough, the bitmap can only hold so much information, then it has to dump out, and a delta sync is then used to verify the integrity of the disk by doing a block check-sum comparison. That’s how it was explained to me anyway.

    one additional question

    the delta sync depends on the bandwidth speed right?

    I mean both but when configuring reverse protection the delta sync takes a long of  time but I assume if  we double the bandwidth between sites this delta sync time will reduce?



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