Administrator Guide

Table 53. Comparing Synchronous Replication and Traditional Replication provides in-depth information about the dierences
between the two features.
Table 53. Comparing Synchronous Replication and Traditional Replication
Consideration Traditional Replication Synchronous 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 dierent pools within the
same PS Series group.
Synchronous replication is useful for maintaining
two copies of a volume’s data in the same data
center, or dispersed to two dierent facilities on
the same campus or in the same metropolitan area.
An advantage of synchronous replication is that it
captures a duplicate copy of every write. One
disadvantage is that if an application writes bad
data to the volume, the bad data is simultaneously
written to both the SyncActive and SyncAlternate
volumes.
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 recongure
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
precongured to use the secondary group storage.
See “Impact on applications” for more information.
If a disaster involving the active pool occurs, you
can manually switch the volume to the alternate
pool.
After the switch, the alternate pool becomes the
active pool and hosts the volume.
Host access to the volume is disrupted by the
switch, but iSCSI initiators do not need to be
recongured.
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 5 minutes.
You can also restore data to the point in time when
any previous replicas were created, provided that
the replicas have been retained.
Synchronous replication provides a single recovery
point: the most recent acknowledged write to the
volume.
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 active and alternate pools,
synchronous replication is sensitive to network
latency.
The network must be able to handle the load of
the data transfer from the active pool to the
alternate pool and complete the replication in a
timely manner, or application performance could
suer.
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.
Synchronous replication creates snapshots of the
volume whenever the SyncActive and
SyncAlternate volumes are switched.
In addition, you can schedule the creation of
snapshots, or create them on demand, as you
would with any other volume.
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About Synchronous Replication