HP Veritas Volume Manager 5.0 -UX 11i v3 Volume read policies

466Performance monitoring and tuning

Performance guidelines

Volume read policies

To help optimize performance for different types of volumes, VxVM supports the following read policies on data plexes:

■round—a round-robinread policy, where all plexes in the volume take turns satisfying read requests to the volume.

■prefer—a preferred-plexread policy, where the plex with the highest performance usually satisfies read requests. If that plex fails, another plex is accessed.

■select—default read policy, where the appropriate read policy for the configuration is selected automatically. For example, prefer is selected when there is only one striped plex associated with the volume, and round is selected in most other cases.

Note: You cannot set the read policy on a RAID-5 data plex. RAID-5 plexes have their own read policy (RAID).

For instructions on how to configure the read policy for a volume’s data plexes, see “Changing the read policy for mirrored volumes” on page 289.

In the configuration example shown in Figure 16-2, the read policy of the mirrored-stripe volume labeled Hot Vol is set to prefer for the striped plex PL1. This policy distributes the load when reading across the otherwise lightly- used disks in PL1, as opposed to the single disk in plex PL2. (HotVol is an example of a mirrored-stripe volume in which one data plex is striped and the other data plex is concatenated.)

Figure 16-2 Use of mirroring and striping for improved performance

HotVol	HotVol	HotVol	HotVol
PL1 SD1	PL1 SD2	PL1 SD3	PL2 SD1
	Lightly	Lightly
Lightly
used	used	used	Lightly
area	area	area
			used
			area

Disk 1 Disk 2 Disk 3 Disk 4