HP StorageWorks HSG80 ACS Solution Software V8.8 for IBM AIX Installation and Configuration Guide (AA-RV1HA-TE, March 2005)

Planning a Subsystem
37HSG80 ACS Solution Software V8.8 for IBM AIX Installation and Configuration Guide
Selecting a Cache Mode
The cache module supports read, read-ahead, write-through, and write-back
caching techniques. The cache technique is selected separately for each unit. For
example, you can enable only read and write-through caching for some units
while enabling only write-back caching for other units.
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.
Subsequent reads for the same data takes the data from cache rather than
accessing the data from the disks. This process is called read caching.
Read caching can improve response time to many of the host’s read requests. By
default, read caching is enabled for all units.
Read-Ahead Caching
During read-ahead caching, the controller 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 units.
Write-Back Caching
Write-back caching improves the subsystem’s response time to write requests by
allowing the controller to declare the write operation complete as soon as the data
reaches cache memory. The controller performs the slower operation of writing
the data to the disk drives at a later time.
By default, write-back caching is enabled for all units, but only if there is a backup
power source for the cache modules (either batteries or an uninterruptible power
supply).