March 2013
Dell EqualLogic Configuration Guide v14.1
6-50
In SyncRep-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.
When you 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. If you delete a
volume for which SyncRep 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.
6.2.3
Synchronous states
There are three SyncRep states for a volume:
•
In-Sync —SyncActive and SyncAlternate pools contain the same volume data.
•
Paused—Administrator has paused SyncRep. While SyncRep 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 SyncRep. 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 SyncRep 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 SyncRep is paused and, when the administrator resumes SyncRep, 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
SyncRep 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
6.2.4
Caveats:
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 SyncRep 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.
6.2.5
Requirements for using SyncRep
Before you can configure a volume to use SyncRep, verify that the following requirements are met:
•
Two pools, each containing at least one member.
