VERITAS Volume Manager 3.5 Administrator's Guide (September 2004)

Performance Monitoring and Tuning
Performance Guidelines
Chapter 12388
Mirroring and striping can be used together to achieve a significant
improvement in performance when there are multiple I/O streams.
Striping provides better throughput because parallel I/O streams can
operate concurrently on separate devices. Serial access is optimized
when I/O exactly fits across all stripe units in one stripe.
Because mirroring is generally used to protect against loss of data due to
disk failures, it is often applied to write-intensive workloads which
degrades throughput. In such cases, combining mirroring with striping
delivers both high availability and increased throughput.
A mirrored-stripe volume may be created by striping half of the available
disks to form one striped data plex, and striping the remaining disks to
form the other striped data plex in the mirror. This is often the best way
to configure a set of disks for optimal performance with reasonable
reliability. However, the failure of a single disk in one of the plexes
makes the entire plex unavailable.
Alternatively, you can arrange equal numbers of disks into separate
mirror volumes, and then create a striped plex across these mirror
volumes to form a striped-mirror volume (see “Mirroring Plus Striping
(Striped-Mirror, RAID-1+0 or RAID-10)” on page 26). The failure of a
single disk in a mirror does not take the disks in the other mirrors out of
use. A striped-mirror layout is preferred over a mirrored-stripe layout for
large volumes or large numbers of disks.
RAID-5
NOTE You may need an additional license to use this feature.
RAID-5 offers many of the advantages of combined mirroring and
striping, but requires less disk space. RAID-5 read performance is
similar to that of striping and RAID-5 parity offers redundancy similar
to mirroring. Disadvantages of RAID-5 include relatively slow write
performance.
RAID-5 is not usually seen as a way of improving throughput
performance except in cases where the access patterns of applications
show a high ratio of reads to writes.