AIX 5L Problem Determination - IBM Redbooks

The Kb_read column
The Kb_read column reports the total data (in KB) read from the physical volume
during the measured interval.
The Kb_wrtn column
The Kb_wrtn column shows the amount of data (in KB) written to the physical
volume during the measured interval.
Taken alone, there is no unacceptable value for any of the above fields, because
statistics are too closely related to application characteristics, system
configuration, and type of physical disk drives and adapters. Therefore, when
you are evaluating data, look for patterns and relationships. The most common
relationship is between disk utilization (%tm_act) and data transfer rate (tps).
To draw any valid conclusions from this data, you have to understand the
application’s disk data access patterns, such as sequential, random, or
combination, as well as the type of physical disk drives and adapters on the
system. For example, if an application reads/writes sequentially, you should
expect a high disk transfer rate (Kbps) when you have a high disk busy rate
(%tm_act). Columns Kb_read and Kb_wrtn can confirm an understanding of an
application's read/write behavior. However, these columns provide no
information on the data access patterns.
Generally, you do not need to be concerned about a high disk busy rate
(%tm_act) as long as the disk transfer rate (Kbps) is also high. However, if you
get a high disk busy rate and a low disk transfer rate, you may have a
fragmented logical volume, file system, or individual file.
Discussions of disk, logical volume, and file system performance sometimes lead
to the conclusion that the more drives you have on your system, the better the
disk I/O performance. This is not always true because there is a limit to the
amount of data that can be handled by a disk adapter. The disk adapter can also
become a bottleneck. If all your disk drives are on one disk adapter, and your hot
file systems are on separate physical volumes, you might benefit from using
multiple disk adapters. Performance improvement will depend on the type of
access.
To see if a particular adapter is saturated, use the iostat command and add up
all the Kbps amounts for the disks attached to a particular disk adapter. For
maximum aggregate performance, the total of the transfer rates (Kbps) must be
below the disk adapter throughput rating. In most cases, use 70 percent of the
throughput rate. In operating system versions later than AIX Version 4.3.3, the -a
or -A option will display this information.
Chapter 10. Performance problem determination 263