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

Planning a Subsystem
1–8 HSG80 ACS Solution Software Version 8.7 for Compaq OpenVMS Installation and
Configuration Guide
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 1–6.
The total memory available for cached data is reduced by half, but the level of
protection is greater.
Figure 1–6: Mirrored caching
Before enabling mirrored caching, make sure the following conditions are met:
• Both controllers support the same size cache.
• Diagnostics indicate that both caches are good.
• No unit errors are outstanding, for example, lost data or data that cannot be written
to devices.
• Both controllers are started and configured in failover mode.
Cache module A Cache module B
A
cache
Copy of
B
cache
Copy of
A
cache
B
cache
CXO5729A