HSG80 ACS Solution Software Version 8.7 for Compaq OpenVMS Installation and Configuration Guide

Planning a Subsystem
1–7HSG80 ACS Solution Software Version 8.7 for Compaq OpenVMS Installation and
Configuration Guide
1–7
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 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).