After creating a VPG from AWS, VPG initial-sync may not progress past 0% and could potentially revert back and forth between x% and 100%.
ZCA AMIs are created by default as size 'm4.xlarge' and due to performance issues with the default 'm4.xlarge' instance size, the minimum required ZCA AMI instance size for Zerto running on version 7.0 is now 'm5.xlarge'. (http://s3.amazonaws.com/zertodownload_docs/Latest/Zerto%20Virtual%20Replication%20AWS%20Enterprise%20Guidelines.pdf),
ZCA performance can be further improved by converting the default 'm4.xlarge' ZCA instance size to 'm5.xlarge' as 'm5.xlarge' is less expensive and provides a newer hardware generation than 'm4x.large'.
Please see the following link for more information on all AWS Instances:
To resolve a VPG that is stuck in an initial-sync state due to the ZCA instance size being 'm4.xlarge':
1) Shutdown the ZCA
2) Change the instance size of the ZCA from m4.xlarge to an m5 class instance size, such as 'm5.xlarge'
3) Power on the ZCA
4) Monitor the initial-sync via the Zerto GUI to ensure the initial-sync completes
NOTE: Should the initial-sync still not progress after the instance size was migrated to an m5 class instance size, please contact Zerto Support to assist with additional troubleshooting.