VERITAS Volume Manager 3.1 Administrator's Guide

Performance Monitoring
Performance Monitoring
Chapter 9 401
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 needto 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. For databases, it may be possible to identify the space used by
a particularly busy index or table. If these can be identified, they are
reasonable candidates for moving to non-busy disks.
Examining the ratio of reads and writes helps to identify volumes that
can be mirrored to improve their performance. If the read-to-write ratio
is high, mirroring could 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, the system
administrator can anticipate system limitations and plan for additional
resources.