Veritas Volume Manager 5.0 Administrator's Guide (September 2006)

280 Administering volumes

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 423.

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

For example, to set the policy for vol01 to read preferentially from the plex vol01-02,

use the following command:

# vxvol -g mydg rdpol prefer vol01 vol01-02

To set the read policy to select, use the following command:

# vxvol [-g diskgroup] rdpol select volume

For more information about how read policies affect performance, see “Volume read

policies” on page 454.