Administrator Guide

Consideration Traditional Replication Synchronous Replication (SyncRep)
See About Synchronous Replication and Snapshots
for more information.
Scheduling Replication operations can be scheduled using the
same mechanism used for scheduling snapshots.
Replication between the SyncActive and
SyncAlternate volumes is continuous. Therefore,
scheduling synchronous replication is not required.
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 active pool and the alternate pool must
have enough space for the volume and snapshot
reserve. Dell recommends that snapshot borrowing
be enabled in groups where synchronous
replication is in use, and that the snapshot reserve
for synchronous replication volumes be set to 100
percent or more of the volume’s size.
Impact on applications iSCSI initiators must be recongured to connect to
the secondary group after the failover, or an
alternate set of host resources must be brought
online, both of which might 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 might be able to restore the volume to a
point in time before the corruption occurred.
Pool switches might cause disruptions in host
access to the volume, but no change to the iSCSI
initiator conguration 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 synchronous replication is rst enabled, or at
any other time when data is being written to both
copies of the volume to become in sync,
performance degradation might occur. This eect
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. The groups' pool
conguration is not a consideration.
One PS Series group containing two storage pools,
each of which must contain at least one member.
How Synchronous Replication Protects Volume Availability in Dierent
Scenarios
The following scenarios describe how synchronous replication (SyncRep) protects volume availability:
• Normal synchronous replication operation
• SyncAlternate volume unavailable
• SyncActive volume unavailable
Normal Synchronous Replication Operation
In normal synchronous replication operation, in which the volume is in sync, the pools containing the SyncActive and SyncAlternate
volumes contain identical data. Volume writes are accepted as shown in Figure 32. Synchronous Replication.
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About Synchronous Replication