Dell PowerVault ME4 Series Storage System Administrator’s Guide

Subsequent replications

The following figure illustrates the internal process that take place in replications subsequent to the initial replication of a single 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.