March 2013
Dell EqualLogic Configuration Guide v14.1
6-44
6.1.3
Replication process
When a replica is created the first replication process completes transfer of all volume data. For
subsequent replicas, only the data that changed between the start time of the previous replication
cycle and the start time of the new replication cycle is transferred to the secondary group. Dedicated
volume snapshots are created and deleted in the background as necessary to facilitate the replication
process.
A volume replica set is defined as follows:
Volume
Replica Set
A full copy of the primary volume,
with data synchronized to the beginning
of the most current completed replication
=
+
A time sequenced set of replicas, where
each replica corresponds to the state of the
volume at the beginning of a prior replication.
The number of prior replicas in the replica set that can stored on the secondary group is limited by the
size of the Replica Reserve allocated for that volume and the amount of data that changes.
Replication processing occurs in a series of phases. The flowchart in Figure 13 shows the process
phases, focusing on how the process tracks and copies changes that occur between each replica
cycle.
Referring to the phases shown in Figure 13:
Replication Setup (one-time):
Configure replication partnership and volume replication settings.
Replication Processing (repeating):
The primary group checks for availability of sufficient delegated and replica reserve space on the
secondary group at the beginning of each replication processing phase. If adequate space is not
available then the process will pause and generate an event message. Replication will continue
once sufficient space is made available. This part of the process is not shown in the chart.
Primary to secondary volume data replication is completed. The process steps vary based on
replication status (first or subsequent) and fast failback mode (enabled or disabled). During this
process Local Reserve is consumed by a hidden snapshot (and the fast failback snapshot if
enabled). Volume data changes that occur during the replication processing phase are stored by
the hidden snapshot in Local Reserve. Replica Reserve allocated to the volume within delegated
space on the secondary group receives all volume data changes. Replica Reserve is consumed by
the most recent complete volume replica plus all prior replicas stored in the replica set.
Between Replication Events (repeating):
Once first replication has occurred the system continues to keep track of volume data changes
that occur so that subsequent replication processes can copy those changes to the replica set.
This tracking process does not consume additional space.
