HP Array Controller HSG V8.7 Software - Manual (Page 66)

2–12 HSG80 User’s Guide
Enabling Mirrored Write-Back Cache
Before configuring dual-redundant controllers and enabling mirroring,
ensure the following conditions are met:
n Both array controllers support the same size cache, 64 MB, 128
MB, 256MB, or 512 MB.
n Diagnostics indicates that both caches are good.
n Both caches have a battery present, if you have not enabled the
CACHE_UPS switch. A battery does not have to be present for
either cache if you enable the CACHE_UPS switch.
n No unit errors are outstanding, for example, lost data or data that
cannot be written to devices.
n Both array controllers are started and configured in failover mode.
For important considerations when adding or replacing DIMMs in a
mirrored cache configuration, see “Replacing DIMMs,” page 5–42.
Selecting a Cache Mode
Before selecting a cache mode you should understand the caching
techniques supported by the cache module.
The cache module supports read, read-ahead, write-through, and write-
back caching techniques that you can enable separately for each storage
unit in a subsystem. For example, you can enable only read and write-
through caching for some units while enabling only write-back caching
for other units.
For details about these caching techniques, see“Caching Techniques,”
page 1–20.
Fault-Tolerance
The cache module supports the following features to protect the
availability of its unwritten (write-back) data:
n Nonvolatile memory (required for write-back caching).
n Dynamic caching techniques (automatic).
For details about these features, see “Fault-Tolerance for Write-Back
Caching,” page 1–21.