cache utilisation for acp_disk mode

Acp_disk allows the Symm to destage local writes to disk (thus freeing up the cache slot), and then will read those changed tracks from the physical disks when they're ready to be copied over the RDF links. In contrast, acp_wp prevents tracks from being destaged to disk until they have been copied over the RDF links.. in cases where there is a write spike, or a congested link between the R1 and R2 frame, this can cause excessive changes to build up in the cache of the R1 frame.

The skew value defines the number of I/Os that can be out of sync between the R1 and the R2 frame. When this value is reached, SRDF reverts to its primary mode, which is nearly always synchronous. The other primary mode is semi-sync, but this is rarely used. If your links revert to sync mode, and your R1 and R2 frames are far apart geographically, or your links have high latency, then your production performance will be affected.

The SRDF/A uses a new cache-based Delta Set architecture to send sets of data to the remote location, significantly reducing the amount of bandwidth required to support the replication operation. Delta Sets maintain an atomic copy of the data at the remote site at all times, ensuring consistent restartable copies are always available.

Delta Sets are cache-resident collections of writes that have occurred within a specific period of time.

Because Delta Sets are collecting the new writes in cache while experiencing locality-of-reference benefits, SRDF/A is able to ride through periods of increased write activity more efficiently, and can be used in conjunction with Delta Set Extension which allows for spillover to disk.