HSG80 ACS Solution Software V8.6 for IBM AIM Installation and Configuration Guide

1–8 HSG80 ACS Solution Software Version 8.6 for IBM AIX Installation and Configuration Guide
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 will take 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 uninterruptable power supply).
Write-Through Caching
Write-through caching is enabled when write-back caching is disabled. When the
controller receives a write request from the host, it places the data in its cache module,
writes the data to the disk drives, then notifies the host when the write operation is
complete. This process is called write-through caching because the data actually passes
through—and is stored in—the cache memory on its way to the disk drives.