36 Planning: RAID Controller Modules
Cache Functions and Features
Cache Mirroring
Cache mirroring function copies accepted host-write data from the primary
controller to the partner controller. This action ensures that host-write data is
safely mirrored to the partner controller before successful completion status is
returned to the host. If a controller fails, the surviving controller safely retains
all mirrored data. Cache mirroring is enabled by default.
Write-Back Cache
In Write-back Cache, write operations result in a completion signal being
sent to the host operating system as soon as the cache receives the data to be
written. The target physical disk receives the data at a more appropriate time
in order to increase controller performance. In dual-active controller
configurations with Write-back Caching enabled, the write data is always
mirrored to the cache of the second controller before completion status is
issued to the host initiator. Write-Back Cache is enabled by default unless
cache mirroring is disabled.
Write-Through Cache
In write-through cache, data is written to the physical disk before completion
status is returned to the host operating system. Write-through cache is
considered more robust than write-back cache, since a power failure is less
likely to cause loss of data. The RAID controller automatically switches to
write-through if cache mirroring is disabled or if the battery is missing or has a
fault condition.
book.book Page 36 Tuesday, September 27, 2011 3:25 PM
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