VERITAS Volume Manager 3.5 Administrator's Guide (September 2002)

Changing the Read Policy for Mirrored Volumes
204 VERITAS Volume Manager Administrator’s Guide
Note Sparse log plexes are not valid. They must map the entire length of the log. If
increasing the log length would make any of the logs invalid, the operation is not
allowed. Also, if the volume is not active and is dirty(for example, ifit has notbeen
shut down cleanly), the log length cannot be changed. This avoidsthe loss of any of
the log contents (if the log length is decreased), or the introduction of random data
into the logs (if the log length is being increased).
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