Veritas Volume Manager 5.0 Administrator's Guide (September 2006)

455Performance monitoring and tuning
Performance monitoring
Performance monitoring
As a system administrator, you have two sets of priorities for setting priorities for
performance. One set is physical, concerned with hardware such as disks and controllers.
The other set is logical, concerned with managing software and its operation.
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 trace data, or both of these (this is the default action).
Selection can be limited to a specific disk group, to specific VxVM kernel I/O object
types, or to particular named objects or devices.