This configuration is useful for performing off-line data mining on recent copies of live data.
The configuration appears as follows:
- Site A contains Cluster A and Storage A.
- Site B has Cluster B, Storage B1 and Storage B2. Between B1 and B2 is a very high-bandwidth mirror link (typically, ShadowImage or TrueCopy); B1 and B2 can be in the same storage subsystem. The mirror relationship between B1 and B2 is broken during office hours. Cluster B loads spans from, and mounts file systems on, Storage B2. No cluster is connected to Storage B1.
- Between Storage A and Storage B1 is a mirror relationship over a medium- or high-bandwidth link: typically, TrueCopy, TrueCopy ED or HUR.
To bring Cluster B up to date with Cluster A
- If the mirror between Storage A and Storage B1 is asynchronous, or near-synchronous, synchronize it. Only changed blocks are copied across the WAN, and so the time taken is acceptable. If the mirror between A and B1 is synchronous, this step is unnecessary, but more bandwidth is required between the two sites in order to maintain acceptable performance at Site A.
- On Cluster B, use the span-prepare-for-resync command.
- Remake the broken mirror relationship between Storage B1 and Storage B2. Because of the high-bandwidth link between these two sets of storage, the time taken to perform the update is acceptable, but synchronization is not instantaneous.
- Once synchronisation is complete, break the mirror link between B1 and B2, bringing B1 and B2 back into the simplex state. B2 now has a copy of the data on B1, which in turn is a copy of the data that was on A at the start of step 1.
- On Cluster B, use the span-reload-after-resync command. If Cluster B mounted file systems on Storage B1, it would write to the storage, even if file systems were mounted read-only. These writes would make it impossible to do an incremental update over the link between Sites A and B. However, the fast link between B1 and B2 makes a complete resynchronization realistic. This is why three copies of the data are required.