LaCie 131018 - StorView Snapshot - Mac, User Manual

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StorView RAID Module
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6.2.1 Configuring Array Writeback Cache
In a writeback cache operation, data is sent to the controller from the host and before actually sending
the data to the drives, the controller immediately responds to the host confirming the data was received
and written to the disk (even though the data may have not been written to the disk). The host may then
send more data. This can significantly increase performance for host systems that only send a low
number of commands at a time. The controller caches the data, and if more sequential data is sent from
the host, it can cluster the writes together to increase performance further. If sufficient data is sent to fill
a stripe in RAID 5, 6, and 50 configurations, the controller can perform a Full Stripe Write, which
significantly reduces the write overhead associated with RAID 5, 6, and 50.
Disabling writeback cache ensures that the data is sent to the drives before status is returned to the host.
With writeback cache enabled, if a short term power failure occurs, the battery back-up unit provides
adequate power to ensure that cache is written to disk when the power is restored.
In duplex operations, the cache is mirrored to both controllers which provides further redundancy in the
event of a single controller failure. Mirrored cache is designed for absolute data integrity. The cache in
each controller contains both primary cached data for the disk groups it owns, and a copy of the primary
data of the other controllers. Mirrored cache ensures that two copies of cache exist on both controllers
before confirming to the operating system that the write operation has completed.
Normally, write-intensive operations will benefit from the higher performance when writeback cache is
enabled on that array. Read-intensive operations, such as a streaming server, may not benefit from
writeback cache.