Administrator Guide

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About Synchronous Replication
Synchronous replication (SyncRep) is the simultaneous writing of data to two pools for a volume in the same PS Series group,
resulting in two hardware-independent copies of the volume data. Each write must go to both pools before the write is
acknowledged as complete. If one copy of the volume data is not available due to a power failure or resource outage, you can still
obtain the data from the other pool.
NOTE:
• You cannot perform traditional replication on a synchronous replication-enabled volume.
• To support up to 32 volumes for synchronous replication, a PS Series group must contain only PS6xx0 arrays. If the
group contains one or more PS4xx0 arrays, only 4 volumes are supported.
How Synchronous Replication Works
Synchronous replication (SyncRep) is enabled on a per-volume basis. For volumes for which synchronous replication is not enabled,
volume data and snapshots are located only in the pool to which the volume is assigned. For synchronous replication-enabled
volumes, volume data exists simultaneously on two copies of the volume:
• SyncActive — The active copy of the volume to which iSCSI initiators connect when reading and writing data.
• SyncAlternate — When data is written to the SyncActive volume, the group simultaneously writes the same data to this copy of
the volume in the alternate pool.
You can switch a volume between being the SyncActive volume and the SyncAlternate volume. The former SyncActive volume
becomes the SyncAlternate volume and vice versa. However, note that if you have more than one volume, but switch only one of
them, only that volume switches from one state to the other. The other volumes retain their current status.
When you switch a volume, no iSCSI target conguration changes are required. During the switch, host connections are logged out;
iSCSI initiators can reconnect when the switch has completed. Depending on its conguration, the initiator might reconnect
automatically.
When synchronous replication is rst enabled, or at any other time when data is being written to both volumes to become in sync,
performance degradation might occur. This eect increases with the quantity of tracked changes, but it is signicantly reduced after
the volumes become in sync. Also, depending on the quantity of tracked changes, activity within the group, and available network
bandwidth, an extended period of time might elapse before the two volumes become in sync again. The Group Manager GUI displays
the status of this operation.
NOTE: If you delete a volume for which synchronous replication is enabled, the group will place the SyncActive volume
into the recovery bin. However, the SyncAlternate volume will be deleted and cannot be recovered.
Compare SyncRep and Traditional Replication
The decision to use synchronous replication (SyncRep) or traditional replication should be driven primarily by your business
requirement and recovery point objectives (RPO). However, you also should examine technical considerations such as networking,
available capacity, and data recovery times when deciding whether traditional replication or synchronous replication is best suited for
protecting your volumes.
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About Synchronous Replication