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

454 Performance monitoring and tuning
Performance guidelines
■ round—a round-robin read policy, where all plexes in the volume take turns
satisfying read requests to the volume.
■ prefer—a preferred-plex read 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 281.
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
Note: To improve performance for read-intensive workloads, you can attach up to 32 data
plexes to the same volume. However, this would usually be an ineffective use of disk
space for the gain in read performance.
Disk 1 Disk 2 Disk 3 Disk 4
HotVol
PL1 SD1
HotVol
PL1 SD2
HotVol
PL1 SD3
Lightly
used
area
Lightly
used
area
Lightly
used
area
Lightly
used
area
HotVol
PL2 SD1