VERITAS Volume Manager 4.1 Administrator's Guide

Performance Monitoring and Tuning
Performance Monitoring
Chapter 12 405
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.