Administrator Guide

Figure 2. Process for subsequent replications
A
User view 1 Step 1: User initiates replication after the first replication has completed.
B Internal view 2 Step 2: S1 contents replace S2 contents.
a Primary system 3 Step 3: Current primary volume contents replace S1 contents.
b Secondary system 4 Step 4: S1 contents replace the secondary volume contents.
c Peer connection 5 Step 5: Differences (delta) between S1 and S2 are identified and replicated over
the peer connection to counterpart S1.
d Primary volume 6 Step 6: S1 contents replace the secondary volume contents.
e Secondary volume
During the initial replication, the entire contents of the primary volume are replicated to the secondary volume. In subsequent
replications, only data that is new or modified since the last replication operation is replicated. This is accomplished by
comparing a snapshot of the primary volume data from the last replication with a current snapshot of the primary volume.
With the exception of this comparison, the process for both the initial and subsequent replications is similar.
During replications subsequent to the initial replication, the following sequence takes place. The user initiates replication on the
primary system (step 1). On the primary system, the S1 contents replace the S2 contents (step 2). (The S2 contents can then
be used for comparison during step 5.) The current primary volume contents replace the contents of S1 on the primary system
(step 3). On the secondary system, the S1 contents replace the S2 contents (step 4). The S1 contents on the primary system,
which match that of the primary volume at the time the replication was initiated, are compared to the S2 contents on the
primary system. Only the data that is the delta between S1 and S2 is replicated to its S1 counterpart on the secondary system,
which is updated with the delta data. The data comparison and replication occur together (step 5). The S1 contents on the
secondary system replace the contents of the secondary volume (step 6). The contents of the primary and secondary volumes
are now synchronized.
Internal snapshots
When first created from the primary volume, the internal snapshots consume very little space but will grow as data is written to
the volume. Just as with any virtual snapshot, the amount of disk space used by an internal snapshot depends on the difference
in the number of shared and unique pages between itself and the volume. The snapshot will not exceed the amount of disk
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Working in the Replications topic