Veritas Volume Manager 5.1 SP1 Administrator"s Guide (5900-1506, April 2011)

If some disks appear to be excessively busy (or have particularly long read or write
times), you may want to reconfigure some volumes. If there are two relatively
busy volumes on a disk, move them closer together to reduce seek times on the
disk. If there are too many relatively busy volumes on one disk, move them to a
disk that is less busy.
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.
Warning: 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.
See “Gathering and displaying I/O statistics” on page 171.
See “Specifying the I/O policy” on page 180.
493Performance monitoring and tuning
Performance monitoring