Dell EqualLogic PS6210XS EqualLogic Group Manager Administrator s Guide PS Ser - Page 305

About Synchronous Replication Volume Collections, SyncRep Failovers

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You can switch the synchronous replication configuration if a failure is imminent for the active pool, or if maintenance needs to be performed on the array hardware in the active pool. You can also switch pools at any time, even if a failure has occurred, provided that the volume is in sync. Aside from the brief period when the volume is offline during the switch, switching eliminates downtime during a maintenance window on the active pool. SyncRep Failovers If the volume is out of sync, the option to switch the pools is replaced with the option to fail over to the alternate pool. You can fail over to the alternate pool only when the volume is offline. As with a pool switch, a pool failover pauses synchronous replication. When the failover is complete, you must manually set the volume online and resume synchronous replication. CAUTION: Failovers should be performed only under extraordinary circumstances, or in cases where you can be sure that the failover will not destroy any data. If you fail over to the SyncAlternate volume while the volume is out of sync, any changes written to the volume since it went out of sync will be written to a snapshot. As with other snapshots, this snapshot can be cloned or restored, but is also subject to deletion by the group's snapshot retention policy. If the snapshot is deleted, its data will be lost and cannot be recovered. When the condition necessitating the switch or failover has been resolved, the administrator can either switch back to the original configuration (if the volume is in sync) or leave the pools in the new configuration. About Synchronous Replication Volume Collections Synchronous replication (SyncRep) volume collections are a unique type of volume collection that can only contain synchronous replication-enabled volumes. As with other volume collections, you can clone or take snapshots of synchronous replication collections. However, synchronous replication collections have the additional capability of supporting synchronous replication operations. In synchronous replication volume collections, all volumes in the collection behave as a group. The volumes simultaneously reside in the same active and alternate pools. When synchronous replication switches are performed on the collection, they take place for all volumes in the collection. You might want to use synchronous replication collections in cases where you have a group of volumes that all must contain data that is current up to the same point in time, such as with an application or host system that is using multiple volumes. If a single volume in the collection goes out of sync, the group takes snapshots of all volumes in the collection, creating a common point in time to which all volumes in the collection can be restored. The following requirements and restrictions apply when working with synchronous replication volume collections: • Synchronous replication collections are considered to be in sync only when all volumes are in sync, and go out of sync if a single volume within the collection ceases to be in sync. • Synchronous replication switches cannot be performed on individual volumes in the collection. • If you create a collection composed of synchronous replication volumes, synchronous replication is not automatically enabled for the collection. You must still enable synchronous replication for the collection. • All volumes in a synchronous replication collection must have synchronous replication enabled before being added to the collection. • Synchronous replication-enabled volumes can be in multiple collections, but they can belong to only one synchronous replication collection at a time. • A volume must be in sync before it can be added to a synchronous replication collection. • You cannot change the pool assignments of volumes in synchronous replication collections. • You cannot assign a volume to a synchronous replication collection while you are changing its pool assignment. • All volumes in a synchronous replication volume collection must have the same active and alternate pools. You cannot add volumes to the collection if they have pool assignments that differ from the assignment specified for the volumes in the collection. To add these volumes to the collection, you must first change their pool assignments so that they match that of the collection. About Synchronous Replication 305

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You can switch the synchronous replication
configuration
if a failure is imminent for the active pool, or if maintenance needs to be
performed on the array hardware in the active pool. You can also switch pools at any time, even if a failure has occurred, provided
that the volume is in sync. Aside from the brief period when the volume is
offline
during the switch, switching eliminates downtime
during a maintenance window on the active pool.
SyncRep Failovers
If the volume is out of sync, the option to switch the pools is replaced with the option to fail over to the alternate pool. You can fail
over to the alternate pool only when the volume is
offline.
As with a pool switch, a pool failover pauses synchronous replication.
When the failover is complete, you must manually set the volume online and resume synchronous replication.
CAUTION: Failovers should be performed only under extraordinary circumstances, or in cases where you can be sure that
the failover will not destroy any data.
If you fail over to the SyncAlternate volume while the volume is out of sync, any changes written to the volume since it went out of
sync will be written to a snapshot. As with other snapshots, this snapshot can be cloned or restored, but is also subject to deletion
by the group’s snapshot retention policy. If the snapshot is deleted, its data will be lost and cannot be recovered.
When the condition necessitating the switch or failover has been resolved, the administrator can either switch back to the original
configuration
(if the volume is in sync) or leave the pools in the new
configuration.
About Synchronous Replication Volume Collections
Synchronous replication (SyncRep) volume collections are a unique type of volume collection that can only contain synchronous
replication-enabled volumes. As with other volume collections, you can clone or take snapshots of synchronous replication
collections. However, synchronous replication collections have the additional capability of supporting synchronous replication
operations.
In synchronous replication volume collections, all volumes in the collection behave as a group. The volumes simultaneously reside in
the same active and alternate pools. When synchronous replication switches are performed on the collection, they take place for all
volumes in the collection.
You might want to use synchronous replication collections in cases where you have a group of volumes that all must contain data
that is current up to the same point in time, such as with an application or host system that is using multiple volumes. If a single
volume in the collection goes out of sync, the group takes snapshots of all volumes in the collection, creating a common point in time
to which all volumes in the collection can be restored.
The following requirements and restrictions apply when working with synchronous replication volume collections:
•
Synchronous replication collections are considered to be in sync only when all volumes are in sync, and go out of sync if a single
volume within the collection ceases to be in sync.
•
Synchronous replication switches cannot be performed on individual volumes in the collection.
•
If you create a collection composed of synchronous replication volumes, synchronous replication is not automatically enabled for
the collection. You must still enable synchronous replication for the collection.
•
All volumes in a synchronous replication collection must have synchronous replication enabled before being added to the
collection.
•
Synchronous replication-enabled volumes can be in multiple collections, but they can belong to only one synchronous replication
collection at a time.
•
A volume must be in sync before it can be added to a synchronous replication collection.
•
You cannot change the pool assignments of volumes in synchronous replication collections.
•
You cannot assign a volume to a synchronous replication collection while you are changing its pool assignment.
•
All volumes in a synchronous replication volume collection must have the same active and alternate pools. You cannot add
volumes to the collection if they have pool assignments that
differ
from the assignment
specified
for the volumes in the
collection. To add these volumes to the collection, you must
first
change their pool assignments so that they match that of the
collection.
About Synchronous Replication
305