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468 Performance monitoring and tuning
Performance monitoring
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.
For detailed information about how to use
vxtrace, refer to the vxtrace(1M)
manual page.
Printing volume statistics
Use the vxstat command to access information about activity on volumes,
plexes, subdisks, and disks under VxVM control, and to print summary statistics
to the standard output. These statistics represent VxVM activity from the time
the system initially booted or from the last time the counters were reset to zero.
If no VxVM object name is specified, statistics from all volumes in the
configuration database are reported.
VxVM records the following I/O statistics:
■ count of operations
■ number of blocks transferred (one operation can involve more than one
block)
■ average operation time (which reflects the total time through the VxVM
interface and is not suitable for comparison against other statistics
programs)
These statistics are recorded for logical I/O including reads, writes, atomic
copies, verified reads, verified writes, plex reads, and plex writes for each
volume. As a result, one write to a two-plex volume results in at least five
operations: one for each plex, one for each subdisk, and one for the volume. Also,
one read that spans two subdisks shows at least four reads—one read for each
subdisk, one for the plex, and one for the volume.
VxVM also maintains other statistical data. For each plex, it records read and
write failures. For volumes, it records corrected read and write failures in
addition to read and write failures.
To reset the statistics information to zero, use the
-r option. This can be done
for all objects or for only those objects that are specified. Resetting just prior to