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APPENDIX D: Optimizing RAID 5 Write Performance www.gateway.com
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RAID 5 sub-array
The difficulty in realizing the maximum performance possible introduces the
concept of a sub-array. Suppose an array consisted of two RAID 5 sets (see the
table on
page 148 ). If these are then striped, the resulting array would appear
as shown below. In this case, in order for a full stripe write to be performed, it
is still only necessary to cluster four write commands together, as opposed to
the seven necessary as indicated below. The array of drives appears as two
separate sub-arrays, each with its own rotating parity.
You can see that the more sub-arrays used, the more likely it is that a full stripe
write occurs, and the higher the performance. It is recommended to use either
four or five drives in a sub-array, for best performance. The following table shows
that even with 15 drives, it is still possible to perform full stripe writes, by
clustering together 4 chunks of data.
Multiple drive failures
In a configuration with multiple sub-arrays, it is possible for the array to sustain
multiple drive failures, provided that there is only one failure in each sub-array.