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1–22
General Description
Fault-Tolerance for Write-Back Caching
The cache module supports nonvolatile memory and dynamic cache policies to
protect the availability of its unwritten (write-back) data.
Nonvolatile Memory
Except for disaster-tolerant supported mirrorsets, the controller can provide write-
back caching for storage units as long as the controller's cache memory is nonvolatile.
In other words, to enable write-back caching, you must provide a backup power
source to the cache module to preserve the unwritten cache data in the event of a
power failure. If the cache memory were volatile—that is, if it didn't have a backup
power supply—the unwritten cache data would be lost during a power failure.
By default, the controller expects to use an ECB as the backup power source for its
cache module. See "External Cache Battery," page 1–29, for more information on the
ECB.
However, if your subsystem is backed up by an uninterruptible power supply (UPS),
you can tell the controller to use the UPS as the backup power source with the
command, SET Controller, and using the CACHE_UPS switch.
Cache Policies Resulting from Cache Module Failures
If the controller detects a full or partial failure of its cache module or ECB, it
automatically reacts to preserve the unwritten data in its cache module. Depending
upon the severity of the failure, the controller chooses an interim caching technique—
also called the cache policy—which it uses until you repair or replace the cache
module.
Table 1–8 shows the cache policies resulting from a full or partial failure of cache
module A in a dual-redundant controller configuration. The consequences shown in
this table are the same for cache module B.
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