VERITAS Volume Manager 3.5 Administrator's Guide (September 2002)

Performance Guidelines
284 VERITAS Volume Manager Administrator’s Guide
Volume Read Policies
To help optimize performance for different types of volumes, VxVM supports the
following read policies on data plexes:
◆ 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 setthe readpolicy on aRAID-5 data plex. RAID-5plexes havetheirown
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 204.
In the configuration example shown in the figure “Use of Mirroring and Striping for
Improved Performance,” 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.)
Use of Mirroring and Striping for Improved Performance
Note To improve performance forread-intensive workloads,you can attach upto 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