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

459Performance monitoring and tuning
Performance monitoring
Use I/O tracing (or subdisk statistics) to determine whether volumes have excessive
activity in particular regions of the volume. If the active regions can be identified, split the
subdisks in the volume and move those regions to a less busy disk.
Caution: Striping a volume, or splitting a volume across multiple disks, increases the
chance that a disk failure results in failure of that volume. For example, if five volumes are
striped across the same five disks, then failure of any one of the five disks requires that all
five volumes be restored from a backup. If each volume were on a separate disk, only one
volume would need to be restored. Use mirroring or RAID-5 to reduce the chance that a
single disk failure results in failure of a large number of volumes.
Note that file systems and databases typically shift their use of allocated space over time,
so this position-specific information on a volume is often not useful. Databases are
reasonable candidates for moving to non-busy disks if the space used by a particularly
busy index or table can be identified.
Examining the ratio of reads to writes helps to identify volumes that can be mirrored to
improve their performance. If the read-to-write ratio is high, mirroring can increase
performance as well as reliability. The ratio of reads to writes where mirroring can
improve performance depends greatly on the disks, the disk controller, whether multiple
controllers can be used, and the speed of the system bus. If a particularly busy volume has
a high ratio of reads to writes, it is likely that mirroring can significantly improve
performance of that volume.
Using I/O tracing
I/O statistics provide the data for basic performance analysis; I/O traces serve for more
detailed analysis. With an I/O trace, focus is narrowed to obtain an event trace for a
specific workload. This helps to explicitly identify the location and size of a hot spot, as
well as which application is causing it.
Using data from I/O traces, real work loads on disks can be simulated and the results
traced. By using these statistics, you can anticipate system limitations and plan for
additional resources.
For information on using the
vxdmpadm command to gather I/O statistics for a DMP node,
path, or enclosure, see “Gathering and displaying I/O statistics” on page 138. You can also
use the
vxdmpadm command to change the I/O load-balancing policy for an enclosure as
described in “Specifying the I/O policy” on page 141.