Gateway 840 VT-100, User Manual

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Chapter 8: Statistics
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Access Size This is the percentage of commands of the size
specified. On occasions, an application may specify a
particular access size, but the operating system can
reduce this. This can lead to performance problems. For
example, on Windows NT, Windows 2000, and
Windows 2003 the maximum command size that the
operating system will allow is 1MB. Additionally, some
HBAs further reduce this value by default. However,
some benchmark programs allow a larger access size
to be specified, resulting in misleading results. By
viewing the statistics on the controller, it is possible to
determine the exact size of commands sent by the host,
and so determine whether the results are appropriate.
The access size, in conjunction with the alignment,
gives an indication of how many drives are involved in
an access. For example, consider a RAID 5/50 array
with a chunk size of 64 K. In this case, a 64 K access
with an alignment of 8 K will actually involve 2 data
drives, since it needs to access some data in the first
drive, and the remaining data in the next drive. This is
clearly inefficient, and could be improved by setting the
alignment to 64 K on the operating system. If that is not
possible, using a larger chunk size can help, as this
reduces the number of accesses that span chunks.
Aligning an access on the same value as the access
size will improve performance, as it will ensure that
there are not multi-chunk accesses for commands that
are smaller than a chunk size.
Statistic Description