Volume Replica Set
6.2 Synchronous replication
6.2.9 SyncActive volume unavailable
If a malfunction occurs in the SyncActive pool, or some other event has occurred causing the volume to go offline, you can safely switch or failover to the SyncAlternate volume by following one of the procedures listed below.
• Volume In Sync: If the volume is in sync, you may switch to the SyncAlternate as documented in the online help. Although host access to the volume is disrupted during the switch, no initiator changes are required.
• Volume Out of Sync: If the volume is out of sync, the group administrator performs the steps outlined in the online help to safely restore access to the volume:
Table 22 below gives a comparison between traditional replication and SyncRep.
Table 22 Comparing SyncRep and traditional replication Replication
consideration Traditional replication SyncRep Typical Use
Case
A point-in-time process that is conducted between two Groups, often in
geographically diverse locations.
Replication provides protection against a regional disaster such as an earthquake or hurricane.
Traditional replication has the advantage of providing multiple recovery points.
A disadvantage of traditional replication is that the state of the data between recovery points is unknown; if any changes are made to the volume since the last replica was created, they could be lost.
A real-time process that keeps two identical copies of volume data in two different pools within the same PS Series group.
SyncRep is useful for maintaining two copies of a volume's data in the same data center, or dispersed to two different facilities on the same campus or in the same metropolitan area.
An advantage of SyncRep is that it captures a duplicate copy of every write. One disadvantage of this is that if an application writes bad data to the volume, the bad data is
simultaneously written to both the SyncActive and SyncAlternate pools.
Recovery Time If a disaster occurs in the primary group, you can promote the replica set on the secondary group to a recovery volume.
After the promotion, you must reconfigure initiators to discover and log in to the iSCSI target now hosted by the secondary group, or switch to an alternate set of server resources that have been preconfigured to use the secondary group storage. See the Impact on Applications row of this table for more information.
If a disaster involving the SyncActive pool occurs, you can manually switch the volume to the SyncAlternate pool.
After the switch, the SyncAlternate pool becomes the SyncActive pool and hosts the volume.
Host access to the volume is disrupted by the switch, but iSCSI initiators do not need to be reconfigured.
Recovery Point The recovery volume contains point-in-time data that is current as of the most recent replica. Replication can be scheduled to take place as frequently as once every five minutes.
You can also restore to the point in time when any previous replicas were created, provided that the replicas have been retained.
SyncRep provides a single recovery point: the most recent
acknowledged write to the volume.
Replication
consideration Traditional replication SyncRep Network
Requirements
Replication requires that the network connection between the primary and secondary groups must be able to handle the load of the data transfer and complete the replication in a timely manner.
Because writes are not
acknowledged until they are written to both the SyncActive and
SyncAlternate pools, SyncRep is sensitive to network latency.
The network must be able to handle the load of the data transfer from the SyncActive pool to the SyncAlternate pool and complete the replication in a timely manner, or application performance may suffer.
Snapshots Replication is functionally similar to
snapshots, creating point-in-time copies of the volume. If the "keep failback" option is enabled, the group creates a "failback snapshot" on the primary group every time a replica is created. This allows for "fast failback" capabilities.
In addition, you can schedule the creation of snapshots, or create them on demand, just as you would with any other volume.
See About SyncRep and Snapshots on page 126 for more information.
SyncRep creates snapshots of the volume whenever the SyncActive and SyncAlternate pools are switched.
Scheduling Replication operations can be scheduled using the same mechanism used for scheduling snapshots.
Replication between the SyncActive and SyncAlternate pools is
continuous. Therefore there is no need or mechanism to schedule SyncRep.
Pool Space Requirements
The primary group must have enough space for the volume reserve and local replication reserve, in addition to any snapshot reserve.
The secondary group must have enough free space delegated to the primary group for the volume reserve and the replicas that record changes to the volume's data over time.
Both the SyncActive pool and the SyncAlternate pool must have enough space for the volume and snapshot reserve.
Replication
consideration Traditional replication SyncRep Impact on
Applications
iSCSI initiators must be reconfigured to connect to the secondary group after the failover, or an alternate set of host
resources must be brought online, both of which may cause application disruptions.
If you are using the Host Integration Tools, you can coordinate replication with host software to quiesce applications on a schedule and create application consistent Smart Copies.
Replication can help protect against the corruption of application data: depending on when the replica occurred and what your replica retention policies are, you may be able to restore the volume to a point in time before the corruption occurred.
Pool switches may cause disruptions in host access to the volume, but no change to the iSCSI initiator
configuration is required to restore access.
Writes must be committed to both pools before they are acknowledged to the host, so the application must be able to tolerate whatever additional delay is caused by the simultaneous writes.
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 is diminished after the volume becomes in sync.
PS Series Group Requirements
Two PS Series groups, each of which must contain at least one member.
One PS Series group containing two storage pools, each of which must contain at least one member.