Hardware manual

Group Administration Volume replication
12–15
To obtain an appropriate replica reserve value, estimate the average volume changes that occur between
replication operations. Then, use this calculation, where 105% is the minimum replica reserve value:
105% + [change rate x (number of replicas to keep -1)]
For example, if you estimate that at most 20% of the volume changes
between replication operations, and you
want to keep three replicas, specify 145% for the replica reserve value.
When replication is ongoing, monitor the number of replic
as for each volume and the replica reserve usage. If more
than the desired number of replicas exist, consider decreasing the replica reserve percentage. If less than the desired
number of replicas exist, consider increasing the replica reserve percentage. A low free replica reserve can indicate
optimal use of replica reserve space, if the desired number of replicas exist.
Guidelines for sizing delegated space
The secondary group administrator delegates space to the primary group when configuring the group as a
replication partner. The secondary group administrator can modify the partner configuration and increase or
decrease delegated space.
Ideally, you should request from the secondary group administrator only enough delegated space to store the
desired
number of replicas for each volume.
Guidelines for sizing delegated space
are as follows:
• Recommended value. Ad
d together the maximum replica reserve space requirements for all the primary group
volumes that you want to replicate to the secondary group and request at least that much delegated space.
If you later decide to replicate additional volumes, the secondary
group administrator might need to increase
the delegated space. See Modifying space delegated to a partner on
page 12-20.
• Space-efficient value. Y
ou might want to request delegated space that is less than the recommended value
described above.
Initially, replica reserve is not fully allocated. Instead, it increases automatically, b
ased on volume usage. This
enables you to over-provision delegated space. When you over-provision delegated space, the total maximum
replica reserve space for all the partner volumes exceeds delegated space.
Warning: If you
over-provision delegated space, a volume’s replica reserve might not be able to increase
automatically or through administrative action, preventing the replication operation from
completing.
For example, assume you are replicating five volumes, and the ma
ximum combined replica reserve needed is 70
GB. If you allocate 50 GB for delegated space, delegated space is over-provisioned by 20 GB. If you specify 70
GB for delegated space, each volume’s replica reserve can increase to its maximum.
After you set up replication, yo
u should monitor delegated space usage. If free delegated space is low and the
replica volume reserve for each replicated volume has not reached its maximum, consider increasing the delegated
space.