Veritas Volume Manager 4.1 Administrator's Guide (HP-UX 11i v3, February 2007)

Chapter 8, Administering Volumes
Changing the Read Policy for Mirrored Volumes
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If a volume is active and its length is being reduced, the operation must be forced using
the -o force option to vxvol. This prevents accidental removal of space from
applications using the volume.
The length of logs can also be changed using the following command:
# vxvol [-g diskgroup] set loglen=length log_volume
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, if it has not been
shut down cleanly), the log length cannot be changed. This avoids the 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 [-g diskgroup] rdpol round volume
For example, to set the read policy for the volume, vol01, in disk group, mydg, to
round-robin, use the following command:
# vxvol -g mydg rdpol round vol01