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 space used by the primary
volume. At most, the two internal snapshots together for each volume may consume twice the amount of disk space as the primary
volume from which they are snapped.
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Working in the Replications topic