The size of the SRL is critical to the performance of replication. This section describes some of the considerations in determining the size of the SRL.
You can use the Volume Replicator Advisor (VRAdvisor) tool to help determine the appropriate SRL size.
See How VRAdvisor works.
If the SRL overflows and SRL protection is not enabled, the RLINK is marked STALE,. Otherwise, if SRL protection is set to autodcm or dcm, the RLINK is disconnected and it goes into DCM mode. Because resynchronization is a time-consuming process and during this time the data on the Secondary cannot be used, it is important to avoid SRL overflows. The SRL size needs to be large enough to satisfy four constraints:
It must not overflow for asynchronous RLINKs during periods of peak usage when replication over the RLINK may fall far behind the application.
It must not overflow while a Secondary RVG is being synchronized.
It must not overflow while a Secondary RVG is being restored.
It must not overflow during extended outages (network or Secondary node).
Regardless of the replication mode, the SRL size must be at least 110 MB. If the size that you have specified for the SRL is less than 110 MB, VVR displays an error message which prompts you to specify a value that is equal to or greater then 110 MB.
To determine the size of the SRL, you must determine the size required to satisfy each of these constraints individually. Then, choose a value at least equal to the maximum so that all constraints are satisfied. The information needed to perform this analysis, presented below, includes:
The maximum expected downtime for Secondary nodes
The maximum expected downtime for the network connection
The method for synchronizing Secondary data volumes with data from Primary data volumes. If the application is shut down to perform the synchronization, the SRL is not used and the method is not important. Otherwise, this information could include: the time required to copy the data over a network, or the time required to copy it to a tape or disk, to send the copy to the Secondary site, and to load the data onto the Secondary data volumes.
If you are going to perform Secondary backup to avoid complete resynchronization in case of Secondary data volume failure, the information needed also includes:
The frequency of Secondary backups
The maximum expected delay to detect and repair a failed Secondary data volume
The expected time to reload backups onto the repaired Secondary data volume