Changing the read policy for mirrored volumes


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.

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

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