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Chapter 3: Creating Disk Arrays
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The choices are Automatic, Disabled, and four pre-determined sizes. Select
Automatic (the default), and recommended setting. It lets the controller
determine the optimum setting.
Disabled will turn off the read-ahead cache.
Choose one of the pre-determined sizes to optimize the read performance
based on your data patterns.
7 Save the configuration changes by selecting Yes, then press ENTER.
Writeback cache
In a writeback cache operation, data is sent to the controller from the host and
the controller immediately responds to the host confirming the data was
received and written to the media. The host can 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
5/50 configurations, the controller can perform a full stripe write, which
significantly reduces the write overhead associated with RAID 5/50.
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 make sure
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 storage
processor contains both primary cached data for the disk groups it owns, and
a copy of the primary data of the other storage processor. Mirrored cache
ensures that two copies of cache exist on both storage processors, before
confirming to the operating system that the write has been completed.
Below is a table from the Gateway 840 Series User Guide, which list the hold-up
times for data for the battery backup unit.
