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289Administering volumes
Changing the read policy for mirrored volumes
# vxassist -g mydg settag myvol "dbvol=table space 1"
Dotted tag hierarchies are understood by the list operation. For example, the
listing for
tag=a.b includes all volumes that have tag names that start with a.b.
The tag names site, udid and vdid are reserved and should not be used. To
avoid possible clashes with future product features, it is recommended that tag
names do not start with any of the following strings: asl, be, isp, nbu, sf,
symc or vx.
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.
siteread Reads preferentially from plexes at the locally defined site.
This is the default policy for volumes in disk groups where site
consistency has been enabled.
See “Administering sites and remote mirrors” on page 431.
split Divides read requests and distributes them across all the
available plexes.
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
To set the read policy to prefer, use the following command:
# vxvol [-g diskgroup] rdpol prefer volume preferred_plex