VERITAS Volume Manager 4.1 Administrator's Guide

Performance Monitoring and Tuning
Performance Monitoring
Chapter 12400
Setting Performance Priorities
The important physical performance characteristics of disk hardware are
the relative amounts of I/O on each drive, and the concentration of the
I/O within a drive to minimize seek time. Based on monitored results,
you can then move the location of subdisks to balance I/O activity across
the disks.
The logical priorities involve software operations and how they are
managed. Based on monitoring, you may choose to change the layout of
certain volumes to improve their performance. You might even choose to
reduce overall throughput to improve the performance of certain critical
volumes. Only you can decide what is important on your system and
what trade-offs you need to make.
Best performance is usually achieved by striping and mirroring all
volumes across a reasonable number of disks and mirroring between
controllers, when possible. This procedure tends to even out the load
between all disks, but it can make VxVM more difficult to administer.
For large numbers of disks (hundreds or thousands), set up disk groups
containing 10 disks, where each group is used to create a striped-mirror
volume. This technique provides good performance while easing the task
of administration.
Obtaining Performance Data
VxVM provides two types of performance information: I/O statistics and
I/O traces. Each of these can help in performance monitoring. You can
obtain I/O statistics using the vxstat command, and I/O traces using the
vxtrace command. A brief discussion of each of these utilities may be
found in the following sections.
Tracing Volume Operations
Use the vxtrace command to trace operations on specified volumes,
kernel I/O object types or devices. The vxtrace command either prints
kernel I/O errors or I/O trace records to the standard output or writes the
records to a file in binary format. Binary trace records written to a file
can also be read back and formatted by vxtrace.
If you do not specify any operands, vxtrace reports either all error trace
data or all I/O trace data on all virtual disk devices. With error trace
data, you can select all accumulated error trace data, wait for new error