Posting this case on behalf of Moti.G:
VPGs started to Bitmap and Delta sync after patches were applied on hosts running the protected VMs which took hours to complete (especially the Delta sync) . Is this an expected behavior?
Thank you for contacting Zerto support.
When a host runs maintenance jobs such as installing more memory, applying new patches, security configuration etc, it is required to place it in maintenance mode, so the ongoing replication won’t be affected. It enters or leaves maintenance mode only as the result of a user request.
By that, the VRA who relies on that host will be automatically shut down, and its virtual machines need to be migrated to another host (either manually or automatically by DRS) or shut down.
In your case, if a host or a VRA was rebooted, the ZVM loses connection to its VRA and the replication gets affected. To overcome it, delta sync is triggered and only data that was written during the host operations is being transferred. In delta sync, Zerto compares the source and target vmdks by MD5 checksums to ensure there are no inconsistencies at the MD5 level of the disk. This is probably the reason the delta sync took long (it is faster than the initial sync, but it checks the whole data of the protected disk. Time may vary depending on the size of the protected volume and sites bandwidth).
For more information regarding Zerto syncs, kindly review our following KB:
For placing a host in maintenance mode without affecting replication, I would recommend reviewing the following KB and our online guide:
Let me know if you have any questions.