Gateway E-842R, User Manual

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Number of outstanding commands
For the controller to successfully cluster commands, there has to be a number
of write commands sent simultaneously. Setting the host to send up to 64
commands should prove adequate. Alternatively, enabling writeback cache has
a similar effect because the controller can cluster sequential commands even
if the host only sends a small number of commands at a time.
Access size
With very small accesses, the controller needs a large number of commands to
cluster together to fill up a full stripe. So, the larger the access size the better.
It is best to use an access size that fills a chunk. Of course, even if a stripe is not
filled up, small sequential writes still benefit from command clustering.
Access alignment
The alignment of a command from a host system is determined by the
command’s address. In an optimal system, a write of one chunk of data would
reside exactly within a chunk on one disk. However, if this is not the case, this
write is split up into two separate writes to two different data drives. This has
a negative effect on performance. To overcome these problems, you can, with
more sophisticated operating systems, set the access size and alignment to an
optimal value.
To get the highest performance from this system, it is necessary to have a
number of stripes being written in parallel. As the array expands, with more and
more drives, the number of commands (and amount of sequential data)
necessary to do this increases.
In the figure above, we can see that seven chunks of sequential data are
necessary to fill a stripe. To have multiple commands active for all disk drives,
this requires more data than for the case with five drives. This number increases
as the number of drives increases. If a large number of drives are used, it may
get difficult to achieve maximum performance because it becomes more
difficult to cluster a large number of commands to achieve a full stripe write.