HP Array Controller HSG V8.7 Software - Manual (Page 44)

1–20 HSG80 User’s Guide
Caching Techniques
The cache module supports the following caching techniques to
increase the subsystem’s read and write performance:
n Read caching
n Read-ahead caching
n Write-through caching
n Write-back caching
Read Caching
When the controller receives a read request from the host, it reads the
data from the disk drives, delivers it to the host, and stores the data in
its cache module. This process is called read caching.
Read caching can decrease the subsystem’s response time to many of
the host’s read requests. If the host requests some or all of the cached
data, the controller satisfies the request from its cache module rather
than from the disk drives. By default, read caching is enabled for all
storage units.
See SET unit number MAXIMUM_CACHED_TRANSFER in
Appendix B, “CLI Commands,” for more details.
Read-Ahead Caching
Read-ahead caching begins when the controller has already processed a
read request, and it receives a sequential read request from the host. If
the controller does not find the data in the cache memory, it reads the
data from the disks and sends it to the cache memory.
The controller then anticipates subsequent read requests and begins to
prefetch the next blocks of data from the disks as it sends the requested
read data to the host. This is a parallel action. The controller notifies the
host of the read completion, and subsequent sequential read requests
are satisfied from the cache memory. By default, read-ahead caching is
enabled for all disk units.