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466 Performance 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-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 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
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