VERITAS Volume Manager 4.1 Administrator's Guide

Performance Monitoring and Tuning
Performance Guidelines
Chapter 12398
RAID-5
NOTE You may need an additional license to use this feature.
RAID-5 offers many of the advantages of combined mirroring and
striping, but requires less disk space. RAID-5 read performance is
similar to that of striping and RAID-5 parity offers redundancy similar
to mirroring. Disadvantages of RAID-5 include relatively slow write
performance.
RAID-5 is not usually seen as a way of improving throughput
performance except in cases where the access patterns of applications
show a high ratio of reads to writes.
Volume Read Policies
To help optimize performance for different types of volumes, VxVM
supports the following read policies on data plexes:
• round—a round-robin read policy, where all plexes in the volume
take turns satisfying read requests to the volume.
• prefer—a preferred-plex read policy, where the plex with the
highest performance usually satisfies read requests. If that plex fails,
another plex is accessed.
• select—default read policy, where the appropriate read policy for
the configuration is selected automatically. For example, prefer is
selected when there is only one striped plex associated with the
volume, and round is selected in most other cases.
NOTE You cannot set the read policy on a RAID-5 data plex. RAID-5 plexes
have their own read policy (RAID).
For instructions on how to configure the read policy for a volume’s data
plexes, see “Changing the Read Policy for Mirrored Volumes” on
page 293.