Administrator Guide

● A replication set for a volume group consumes two internal volume groups if the queue policy is set to Discard, or three if
the queue policy is set to Queue Latest. Each internal volume group contains a number of volumes equal to the number of
volumes in the base volume group.
Internal snapshots and internal volume groups count against system limits, but do not display.
Using a volume group for a replication set enables you to make sure that multiple volumes are synchronized at the same time.
When a volume group is replicated, snapshots of all of the volumes are created simultaneously. In doing so, it functions as
a consistency group, ensuring consistent copies of a group of volumes. The snapshots are then replicated as a group. Even
though the snapshots may differ in size, replication is not complete until all of the snapshots are replicated.
For a replication set, the term primary refers to the source volume and the system in which it resides, and the term
secondary is used for the remote copy and the system in which it resides. The secondary volume is meant to be an exact
copy of the primary volume from the last time that replication occurred. To guarantee that the contents from that point in time
match, the secondary volume cannot be mapped, rolled back, or modified except through replication.
While you cannot modify the secondary volume, you can create a snapshot of the secondary volume that you can map, roll back,
and otherwise treat like any volume or snapshot. You can regularly take snapshots to maintain a history of the replications for
backup or archiving, or enable snapshot history for the replication set. These snapshots also can be used in disaster recovery.
For more information on replication sets, see “Creating a replication set from the Replications topic, Creating a replication set
from the Volumes topic, Modifying a replication set, and Deleting a replication set.
Replication process
After you create a peer connection and replication set, you can then replicate volumes between the systems. The initial
replication differs slightly from all subsequent replications in that it copies all of the allocated pages of the primary volume to
the secondary volume. Depending on how large your source volume is and the speed of the network connection, this initial
replication may take some time.
Subsequent replications are completed by resetting one of the hidden snapshots to contain the contents last replicated and
then resetting the other hidden snapshot to the current primary volume contents and comparing the changes. The system
writes any changes it finds on the hidden primary snapshot to the hidden secondary snapshot, after which the secondary
volume is updated to contain the contents of the secondary volume.
The progress and status of the initial and subsequent replications are tracked and displayed. The timestamps for replication
reflect the time zones of the respective systems. When viewed on a secondary system in a different time zone, for example,
replication information will reflect the time zone of the secondary system. For more information on replicating, see Stopping
a replication, Initiating or scheduling a replication from the Replications topic, Initiating or scheduling a replication from the
Volumes topic, Resuming a replication, and Suspending a replication.
You can initiate a replication manually or by using a schedule. When creating a schedule for a replication set, you cannot specify
for replication to occur more frequently than once an hour. For more information on scheduling a replication set, see Initiating or
scheduling a replication from the Replications topic and Initiating or scheduling a replication from the Volumes topic.
Initial replication
The following illustration shows the internal processes that take place during the initial replication of a single volume.
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Working in the Replications topic