HSG80 ACS Solution Software Version 8.7 for Compaq Tru64 UNIX Installation and Configuration Guide

Planning a Subsystem
1–10 HSG80 ACS Solution Software Version 8.7 for Compaq Tru64 UNIX 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.
Enabling Mirrored Caching
In mirrored caching, half of each controller’s cache mirrors the companion controller’s
cache, as shown in Figure 1–8.