VERITAS Volume Manager 4.1 Administrator's Guide

Administering Volumes
Changing the Read Policy for Mirrored Volumes
Chapter 8 293
Changing the Read Policy for Mirrored
Volumes
VxVM offers the choice of the following read policies on the data plexes in
a mirrored volume:
• round—reads each plex in turn in “round-robin” fashion for each
nonsequential I/O detected. Sequential access causes only one plex to
be accessed. This takes advantage of the drive or controller
read-ahead caching policies.
• prefer—reads first from a plex that has been named as the preferred
plex.
• select—chooses a default policy based on plex associations to the
volume. If the volume has an enabled striped plex, the select option
defaults to preferring that plex; otherwise, it defaults to round-robin.
The read policy can be changed from round to prefer (or the reverse), or
to a different preferred plex. The vxvol rdpol command sets the read
policy for a volume.
NOTE You cannot set the read policy on a RAID-5 volume. RAID-5 plexes have
their own read policy (RAID).
To set the read policy to round, use the following command:
# vxvol rdpol round volume
For example, to set the read policy for volume vol01 to round-robin, use
the following command:
# vxvol rdpol round vol01
To set the read policy to prefer, use the following command:
# vxvol rdpol prefer volume preferred_plex
For example, to set the policy for vol01 to read preferentially from the
plex vol01-02, use the following command:
# vxvol rdpol prefer vol01 vol01-02
To set the read policy to select, use the following command: