Disk Array Caching - HP Surestore Disk Array 12h - And FC60 Service Manual

Disk Array Caching

Disk caching is the technique of storing data temporarily in RAM while performing I/Os to
the disk array. Using RAM as a temporary storage medium can significantly improve the
response time for many types of I/O operations. From the host's perspective the data
transfer is complete, even if the disk media was not involved in the transaction. Both write
caching and read caching are always enabled.
Caching enhances disk array I/O performance in two ways:
Read I/O If a read I/O requests data that is already in read cache, the disk array
services the request from cache RAM, thus avoiding the much slower
process of accessing a disk for the data. A pre-fetch capability
enables the disk array to anticipate needed data (for example, on a
file transfer) and read it from disk into the read cache, which helps
significantly with sequential read I/Os.
Write I/O During a write I/O, the disk array writes the requested data into write
cache. Rather than writing the modified data back to the disk
immediately, the disk array keeps it in cache and informs the host
that the write is complete. If another I/O affects the same data, the
disk array can update it directly in cache, avoiding another disk
write. Data is flushed to disk at regular intervals (10 seconds) ro
when the cache flush threshold is reached.
Write cache is mirrored between the two disk array controllers. Each controller maintains
an exact image of the write cache on the other controller. If a controller fails, its write
cache content is flushed to the disk by the remaining controller. Because write cache is
mirrored, the operational controller automatically disables write caching until the failed
controller is replaced. After it is replaced, the operational controller automatically re-
enables write caching. Mirroring effectively reduces the size of available cache by half. A
controller with 256 Mbytes of cache will use half of the memory to mirror the other
controller, leaving only 128 Mbytes for its own cache.
The write cache contents cannot be flushed when both controllers are removed from the
disk array simultaneously. In this case the write cache image is lost and data integrity on
the disk array is compromised. To avoid this problem, never remove both controllers from
the disk array simultaneously.
Capacity Management Features
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