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When the SRL overflows for a particular Secondary, the RLINK corresponding to that Secondary is marked STALE and becomes out of date until a complete resynchronization with the Primary is performed. 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 synchronized.
It must not overflow while a Secondary RVG is restored.
It must not overflow during extended outages (network or Secondary node).
To determine a size of the SRL, you must determine the size that is required to satisfy each of these constraints individually. Choose a value at least equal to the maximum so that all constraints are satisfied. To perform this analysis, you need the following information:
The maximum expected downtime for Secondary nodes
The maximum expected downtime for a network connection
The method for synchronizing Secondary data volumes with data from Primary data volumes. If the application is shutdown to perform the synchronization, the SRL is not used and the method is not important. Otherwise, this information can include the time that is required to copy data over the network or 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.
Note: |
If Automatic Synchronization option is used to synchronize the Secondary, the above-mentioned step is not a concern. |
To perform Secondary backups to avoid complete resynchronization in case of Secondary data volume failure, the following information is required: