How to Interpret Zerto Syncs

Summary

Syncs are generally needed when the recovery volume data is missing, lacking, inconsistent, or needs to be updated after a period of time where replication wasn’t active.
In this article you will find a high level overview of the 3 types of syncs, their triggers, and implications with DR activity.
The syncs are also rated from the lightest and fastest to the heaviest and slowest.

Steps

Initial Sync:

When does it happen:
Initial sync occurs once in a lifetime of a VPG, during its creation.
What data is migrated:
All data residing on the source site protected VMs is being copied to the recovery site in order to create the recovery volume the VPG will be based upon.
Expected time and factors:
Since this sync copies all data from site to site it's expected to be the slowest of all syncs. Time may vary depending on sites bandwidth and total size to be migrated.
Implications on DR activity:
During initial sync, no DR activity is available. Check points are not generated to journal and failover isn’t possible.

Delta Sync: 

When does it happen:
Delta sync occurs during pre-seed, after a live failover with reverse protection enabled, and in cases where the production host was down.
What data is migrated:
Only new data to the protected disks is migrated. 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.
Expected time and factors:
Delta sync 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.
Implications on DR activity:
During delta sync no DR activity is available. Check points are not generated to journal and failover isn’t possible.

Bitmap Sync: 

When does it happen:
Bitmap sync occurs when there are insufficient resources to maintain replication of the I/O load of the protected application. This can occur following WAN failure, in the presence of a network bottleneck, or if hosts or storage involved in replication are experiencing performance-related issues or disconnections
What data is transferred:
Only relevant data that has been changed during the disconnection is transferred. Zerto Virtual Replication uses a smart bitmap which tracks and records the source storage areas that changed during the time in which replication could not have been maintained.
Expected time and factors:
Since no overall check is being made and only the relevant data is being transferred between sites, this is expected to be the fastest sync. Time may vary depending on the amount of data generated while the disconnection occurred and site bandwidth.
Implications on DR activity:
During Bitmap Sync DR activity is available. However, check points are not generated to journal.