Changing the read policy for mirrored volumes


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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 435.

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