When a new Secondary is added to an RDS, its data volumes must be synchronized with those of the Primary unless the Primary and the Secondary data volumes have been zero initialized and the application has not yet been started. You also need to synchronize the Secondary after a Secondary data volume failure, in case of SRL overflow, or after replication is stopped.

This section applies if you choose not to use the automatic synchronization method to synchronize the Secondary. Also, this constraint does not apply if you choose to use a method other than automatic synchronization and if the application on the Primary can be shut down while the data is copied to the Secondary. However, in most cases, it might be necessary to synchronize the Secondary data volumes with the Primary data volumes while the application is still running on the Primary.

During the synchronization period, the application is running and data is accumulating in the SRL. If the SRL overflows during the process of synchronization, the synchronization process must be restarted. Thus, to ensure that the SRL does not overflow during this period, it is necessary that the SRL be sized to hold as much data as the application writes during the synchronization period. After starting replication, this data is replicated and the Secondary eventually catches up with the Primary.

Depending on your needs, it may or may not be possible to schedule the synchronization during periods of low application write activity. If it is possible to complete the synchronization process during a period of low application write activity, then you must ensure that the SRL is sized such that it can hold all the incoming writes during this period. Otherwise, the SRL may overflow.

If however there is an increase in the application write activity then you may need to resize the SRL even when the synchronization is in progress.

If it is not possible to complete the synchronization process during periods of low application write activity, then size the SRL such that it uses either the average value, or to be safer, the peak value.