InfoScale™ 9.0 Replication Administrator's Guide - AIX

How VVR creates instant space-optimized snapshots

This section explains how VVR creates and manages the space-optimized snapshots.

In the following illustration, the Primary has two data volumes: Data Volume 1 and Data Volume 2. For this example we have indicated two specific blocks, namely, A and C in Data Volume 1 and B and D in Data Volume 2.

The Secondary has two data volumes, Data Volume 1 and Data Volume 2 that have all the replicated data from the Primary, including the specified blocks. The Secondary illustrates an instant space-optimized snapshot in which the data resides on the original volume itself. A Read operation to the snapshots will be redirected to the source volumes and writes will result in a copy-on-write operation. The data will be copied to the snapshots only if there is a write to the original data. Because the snapshots are space-optimized the data will actually get written to the cache object only if there is a write to the original data.

Figure: Example 1 - How VVR creates instant space-optimized snapshots

The following illustration indicates the scenario where the Primary receives some updates to the blocks A and B. These are now represented as A' and B'.

The Secondary also receives the updates A' and B'. The write to the data volumes first results in a copy-on-write on to the space-optimized snapshot. A space-optimized snapshot is created on a cache object that holds all the data for the snapshots. Hence during a copy-on-write, the blocks A and B get written onto the cache object, before the changed blocks are written to the original volumes, Data Volume 1 and Data Volume 2. The cache object stores the blocks persistently in the cache volume after remapping the original offsets.

Figure: Example 2 - How VVR writes to the instant space-optimized snapshots

The following illustration indicates the scenario when there is a write from the application to the block C on the snapshot. This block in now indicated as C'. The changed information for the block C is preserved on the cache object using the copy-on-write mechanism described earlier. If there is a read then the changed block C' will be read from the cache object. Similarly, if there is a request for reading block A from the snapshot volume, it will be fetched from the cache object, where it has been copied earlier.

Figure: Example 3 - How VVR reads the changed block from the cache object