HP Veritas Volume Manager 5.0 -UX 11i v3 manual Changing the read policy for mirrored volumes

Administering volumes 289

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 [-gdiskgroup] rdpol prefer volume preferred_plex