Hardware manual

Group Administration Volume replication
12–6
How volume changes affect replication space
How much space you need for replication depends on the volume size and the rate of volume changes.
The first replication of a volume copies the entir
e volume contents from the primary group to the secondary group.
Subsequent replication operations transfer only the data that changed since the previous replication. Replication
time and space requirements increase as the amount of transferred data increases.
It can be difficult to estimate the rate of volume changes be
cause volume usage can vary. Therefore, it can be
difficult to estimate replication time and space requirements. For example:
• Although some applications perform a cons
istent number of volume writes, others have a workload that
changes daily. Therefore, one replication operation might transfer little data and complete quickly, while
another replication might transfer a large amount of data and take a long time.
• In some cases, a volume might appear to have few changes, but the
transferred data is relatively large. Random
writes to a volume can result in a large amount of transferred data, even if the actual data changes are small.
• Some disk operations, such as defragmenting a disk or reorganizing a da
tabase, can increase the amount of
transferred data. However, the defragmentation or reorganization can make subsequent replications more
efficient.
In addition, because volume usage can change
over time, replication space that was adequate for one workload
might become inadequate when you add more users.
If a replication operation requires copying a lar
ge amount of data, you might want to use manual transfer
replication. See About manual transfer r
eplication on page 12-3.
For each replication operation, you can display the amount of
data that the primary group is transferring to a
replication partner. See Displaying replication activity and r
eplicas for a volume on page 12-28. You can also
display the replication history for a volume and the amount of data transferred for each replicati
on operation.
Best practice for replicating volumes
To help ensure successful replication, for each volume that you want to replicate, follow these steps to set up your
replication environment:
1. Plan the volume replication configuration:
a. Gather the following information to help
you determine how much replication space you need:
• Number of replicas you want to keep and the average time span between
each consecutive replica
• Reported size of the volume
• Whether thin-provisioned
• Estimated rate of volume changes (depends on volume usage)
b. Make sure that the primary group has enough free pool
s
pace for the local replication reserve for each
replicated volume. See About local replication reserve on
page 12-8.
c. Identify a replication partner (secondary group) to store the volume replicas. Th
is secondary group must
meet the space and network connectivity requirements in Replication partner requirements on page 1
2-16.