Delta Sync Question

  • This morning we put a few hosts into Maintenance Mode. At that time, Zerto started Delta Syncing volumes. Maintenance on those hosts has been completed.

    We’d like to know if we should wait until the Delta Syncs are complete or if we can put the hosts back into production? If we do put the hosts back into production, what will happen with the syncing VPGs… will they continue until complete or will the sync stop.

    Also, is there an article or brief description you could provide on the differences between different types of syncs (Delta, Initial, Bitmap, etc).

    Thank you for your time


    If it’s possible to wait for the delta sync to complete, that would be better.

    Heres a few explanations about the Syncs.

    the Delta Sync uses a checksum comparison to minimize the use of network resources. A Delta Sync is used when the protected virtual machine disks and the recovery disks should already be synchronized, except for a possible few changes to the protected disks, for example:

    ■ When a virtual machine was added to the VPG and the target recovery disk is defined as a preseeded disk.
    ■ After a source VRA upgrade of a major release: Depending on the nature of the upgrade, a VRA upgrade on the protected side may trigger either a Delta Sync or a Bitmap Sync. See the version release notes to determine if a sync will be triggered with a source VRA upgrade.
    ■ For reverse protection after a move or failover.
    ■ A Force Sync operation was manually initiated on the VPG.
    ■ A host protecting virtual machines was restarted and the protected virtual machines on the host had not been vMotioned to other hosts in the cluster or a protected virtual machine was vMotioned to another host without a VRA, and then vMotioned back to the original host.

    Whereas, Bitmap syncs:
    A change tracking mechanism of the protected machines during a disconnected state or when a VRA buffer is full. In these situations, Zerto Virtual Replication starts to maintain a smart bitmap in memory, in which it tracks and records the storage areas that changed. Since the bitmap is kept in memory, Zerto Virtual Replication does not require any LUN or volume per VPG at the protected side.
    Note: The VRA buffer is set via the Amount of VRA RAM value, specified when the VRA is installed
    The bitmap is small and scales dynamically, containing references to the areas of the protected disk that have changed but not the actual I/O. The bitmap is stored locally on the VRA within the available resources. For example, when a VRA goes down and is then rebooted.
    When required, Zerto Virtual Replication starts to maintain a smart bitmap in memory, to track and record storage areas that change. When the issue that caused the bitmap sync is resolved, the bitmap is used to check updates to the protected disks and send any updates to the recovery site. A bitmap sync occurs when any of the following conditions occur:
    ■ Synchronization after WAN failure or when the load over the WAN is too great for the WAN to handle, in which case the VPGs with the lower priorities will be the first to enter a bitmap sync.
    ■ When there is storage congestion at the recovery site, for example when the VRA at the recovery site cannot handle all the writes received from the protected site in a timely fashion.
    ■ When the VRA at the recovery site goes down and is then rebooted, for example during a Zerto Virtual Replication upgrade.

    The initial sync occurring in order to create the mirror volume on the recovery side once a volume is being protected in a VPG.


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