Hardware manual

Group Administration Volume replication
12–13
When you configure a volume for replication, you specify the replica reserve size as a percentage (minimum
105%) of the replica volume reserve, which approximates in-use volume space. As volume usage increases, the
replica volume reserve increases; therefore, the replica reserve also increases, providing more space for replicas, up
to a limit. See Replica volume reserve.
For example, if you specify 200% for the
replica
reserve, and the replica volume reserve is 2.5 GB, the replica
reserve size is 5.0 GB. If the replica volume reserve increases to 6.0 GB, the replica reserve size increases to 12.0
GB.
The replica volume reserve has a maximum size (the reported volume
size for volumes that are not thin-
provisioned, or the maximum in-use space value for thin-provisioned volumes); therefore, the replica reserve has a
maximum size.
To properly size replica reserve, you must understand how replica reserve space is used. See Replica reserve usage
– first replication and Replica reserve usage – subsequent replications.
Replica reserve usage – first replication
1. The primary group determines how much volume data to replicate. For the first replication operation, the
primary group must copy all the volume data.
2. The primary group increases the replica reserve if the replica volume rese
rve increased since you enabled
replication on the volume.
Note: If there is
not enough free delegated space for the replica reserve increase, the primary group generates
an event message, and the replication pauses
. Replication continues automatically once there is
sufficient delegated space.
3. The primary group copies the contents of the volume to replica reserve, decreasing the
amount of free replica
reserve. For example, if the volume consists of 10 GB of data, free replica reserve decreases by 10 GB.
At this point, the replica reserve contains one replica, whic
h is the most recent complete replica.
Replica reserve usage – subsequent replications
1. The primary group determines how much volume data to replicate. For replication operations other than the
first, the primary group copies only the data that changed since the previous complete replication (the deltas).
2. The primary group increases the replica reserve if the replica volume rese
rve increased since the previous
replication operation.
Note: If there is
not enough free delegated space for the replica reserve increase, the primary group generates
an event message, and the replication pauses
. Replication continues automatically once there is
sufficient delegated space.
3. If there is not enough free replica reserve for the volume data, the primary group deletes
the oldest replica to
free space for the new replica. For example, if the data transfer consisted of 5 GB of new data, the replica
reserve must have 5 GB of free space to store this data.
Because each replica is a representation of volume data at the
time the replication started, only the data that
differentiates the oldest replica from the other replicas is deleted. Therefore, multiple replicas might be deleted
to free sufficient space.