Hardware manual

Group Administration Volume replication
12–14
The most recent complete replica is never deleted automatically, ensuring that you always have a viable copy
of volume data on the secondary group.
Note: If you cannot free
enough replica reserve for the volume data by deleting replicas, the replication
pauses, and the primary group generates an event message, indicating the replica reserve percentage
required to complete the replication.
4. The primary group copies the volume data to
replica reserve, decreasing the amount of free replica reserve. For
example, if the replication transferred 5 GB of new data, free replica reserve decreases by 5 GB.
When each replication completes, the new replica becomes the most rec
ent complete replica.
To make sure that replication operations complete and keep the desired
number of volume replicas, it is important
to allocate sufficient delegated space and specify the correct replica reserve percentage. The amount of delegated
space you need depends on the replica reserve requirements for all the replicated volumes from a partner.
See Guidelines for sizing replica reserve for a volume on pa
ge 12-14 and Guidelines for sizing delegated space on
page 12-15.
Guidelines for sizing replica reserve for a volume
To determine the amount of space that the secondary group must delegate to the primary group, you must obtain
the replica reserve requirement for each primary group volume that you are replicating to the secondary group.
When you configure a volume for replication, you s
pecify the replica reserve size as a percentage (minimum
105%) of the replica volume reserve, which approximates in-use volume space.
As volume changes occur, the replica volume reserve increase
s; therefore, the replica reserve increases, providing
more free space for replicas, up to a limit. If there is insufficient free replica reserve for a new replica, the oldest
replicas are deleted to increase free space.
Note: Replica reserve can increase automatically or by administra
tor action only if free delegated space is
available. See Guidelines for sizing delegated space on pa
ge 12-15.
Ideally, you want to allocate only enough replica reserve to store
the desired number of replicas. In general, the
higher the replica reserve percentage, the more replicas you can store. However, specifying a high percentage for
all volumes might not be the most efficient use of delegated space.
The optimal value for replica reserve depends on the volume ch
ange rate, which can be difficult to estimate, and
the replication frequency. See How volume changes affect re
plication space on page 12-6.
Guidelines for sizing replica reserve are as follows:
• Recommended value. D
ell recommends that you specify 200% for the replica reserve. Specifying 200%
guarantees that you can store at least two replicas, assuming there is sufficient delegated space for the replica
reserve to reach its maximum size. If the replica reserve cannot reach its maximum size due to lack of
delegated space, you are not guaranteed two replicas.
If you want to guarantee more than two replicas
, spec
ify a higher percentage.
• Space-efficient value. For
volumes that are not frequently modified, you might be able to keep the desired
number of replicas by using a replica reserve value that is less than 200%.