How Synchronous Replication Works

Synchronous replication is enabled on a per-volume basis. In volumes for which synchronous replication (SyncRep) is not enabled, volume data and snapshots are located only in the pool to which the volume is assigned. In synchronous replication-enabled volumes, volume data exists simultaneously in two pools:

SyncActive - The pool to which iSCSI initiators connect when reading and writing volume data.
SyncAlternate - When volume data is written to the SyncActive pool, the group simultaneously writes the same data to this pool. You can switch the roles of the SyncActive and SyncAlternate pools.

You can switch the SyncActive and SyncAlternate pools. The former SyncActive pool then becomes the SyncAlternate pool, and vice-versa. No iSCSI target configuration changes are required. During the switch, host connections are logged out. iSCSI initiators can reconnect when the switch has completed. Depending on its configuration, the initiator may automatically reconnect.

Note:  If you delete a volume for which synchronous replication is enabled, the system will place the SyncActive volume into the Recovery Bin. However, the SyncAlternate volume will be deleted outright and cannot be recovered.

Synchronous Replication States

There are three synchronous replication states for a volume:

in sync—SyncActive and SyncAlternate pools contain the same volume data.
paused—Administrator has paused synchronous replication. While synchronous replication is paused, the volume is still online, and initiators can connect to and write to the SyncActive volume. An administrator may pause and later resume synchronous replication. For example, this could happen in a maintenance window during which the SyncAlternate pool is taken offline. If data is written to the volume while synchronous replication is paused, it is written only to the SyncActive pool, and the two pools are out of sync. The group tracks all volume writes while synchronous replication is paused and, when the administrator resumes synchronous replication, writes the tracked changes to the SyncAlternate pool.
out of sync—SyncActive pool and SyncAlternate pool may not contain the same volume data; the SyncActive pool contains the most recent volume data. A volume can become out of sync if synchronous replication is paused, or if one of the pools becomes unavailable or has no free space. The volume can become out of sync when the snapshot reserve in the SyncAlternate pool is full, but only when the snapshot space recovery policy sets volumes offline when the snapshot reserve is depleted.

Whenever the volume's state changes to paused or out of sync, the group creates a snapshot of the volume that reflects the volume's contents at the point in time when the state changed. This snapshot resides in the SyncActive pool. If there is insufficient room in the snapshot reserve for the snapshot, the group does not create the snapshot.

Notes:  Depending on the quantity of tracked changes, activity within the group, and available network bandwidth, there may be an extended period of time before the two pools become in sync again. The Group Manager GUI displays the status of this operation.

When synchronous replication is first enabled, or at any other time when the volume is writing data to both pools to become in sync, performance degradation may occur. This effect increases with the quantity of tracked changes, but it is significantly reduced after the volume becomes in sync.

Requirements for Using Synchronous Replication

Before you can configure a volume to use synchronous replication, verify that the following requirements are met:

Two pools, each containing at least one member.
Adequate network bandwidth between pools.
Free space in each pool to accommodate the volume and snapshot reserve for the volume.
You cannot enable synchronous replication on a volume for which traditional replication is configured, and you cannot enable traditional replication on a volume for which synchronous replication is configured. See the "Disabling Replication" topic in the online help for instructions on disabling traditional replication on a volume.
You cannot enable synchronous replication on a volume that is bound to a group member. See "Binding and Unbinding Volumes to Members" in the Group Manager online help for more information about binding and unbinding volumes.

Related Topics

About Synchronous Replication and Snapshots
About Synchronous Replication Switches and Failovers
Configuring Synchronous Replication (SyncRep) for a Volume

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