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Write-Backs
Write-
Throughs
Environments with a large number of sequential reads should
generate a high number of Read-Ahead Hits relative to Total
Sectors. These hits reduce the number of seek operations and
increase performance. The Read-Ahead Hits count can be increased
by adding more cache memory to the controller. A high percentage
of 4-KB or smaller I/O operations also indicates that increasing the
controller cache would be beneficial.
If cache hits are low, adding more cache RAM can also increase the
hit count. Systems with a large number of disk writes also derive
significant performance benefits from the controller cache.
For RAID 0 and RAID 5 arrays, the default stripe size is set for
optimal performance in most environments. However, some
specific environments can benefit from smaller or larger stripes.
For example, when a RAID 5 write operation accesses all drives,
many stripe crossings occur. By using a larger stripe size, the
RAID 5 parity data can be generated more efficiently. This results
in better write performance. However, if the write crosses one or
more stripes but does not involve all of the drives, performance is
reduced by the larger stripe size.
You should select the stripe size relative to both the I/O segment
size and the number of drives in the array, so that most I/O
operations either
Do not cross stripes and involve only a single drive, or
■
Cross many stripes and involve all the drives in the array.
■
The number of sectors written to disk that were held
in the controller cache and written some time after
the host Write command reported as completed.
The number of sectors written directly to disk before
the Write command ended.
Storage Manager
3-49
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