Gateway E-842R, User Manual

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APPENDIX C: Statistics www.gateway.com
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Statistic Description
Command Size Expressed in the percentage of commands whose size is
specified for reads and writes. The values are displayed
with a horizontal bar for each value. The lack of a bar
displayed for a specific value indicates it is 0% (or less than
1%).
For example, consider a read or write command from a
host system with Logical Block Address (LBA) 0x0000070,
and access size 0x80, expressed in decimal 128. Using 512
byte blocks on the disk drives, this is a read of 64 Kbytes,
which is the command size.
Alignment This is the percentage of commands whose address is
aligned on the specified chunk boundary. 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.
How to Use Command Size and
Alignment To calculate the alignment, we check the LBA for the
largest number of blocks that are evenly divided into it,
in powers of 2. So, we can see that in this case, the
alignment is 0x10 = 16 blocks. This equates to 8 K.
The alignment, in conjunction with the access size, gives
an indication of how many drives are involved in an
access. In the above example, consider a RAID 5/50 array
with a chunk size of 64 K. In this case, the above access
actually involves 2 data drives, since it needs to access 8 K
in the first drive (0x80 – 0x70 = 0x10 blocks = 8 K), and the
remaining 56K in the next drive (0x70 blocks = 56 K). This
is 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, because this
reduces the number of accesses that span chunks.
Aligning an access on the same value as the access size
improves performance because it ensures that there are
no multi-chunk accesses for commands that are smaller
than a chunk size.