Veritas Volume Manager 5.1 SP1 Administrator"s Guide (5900-1506, April 2011)

Combining mirroring and striping
When you have multiple I/O streams, you can use mirroring and striping together
to significantly improve performance.
Because parallel I/O streams can operate concurrently on separate devices, striping
provides better throughput. When I/O fits exactly across all stripe units in one
stripe, serial access is optimized.
Because mirroring is generally used to protect against loss of data due to disk
failures, it is often applied to write-intensive workloads. This approach degrades
throughput. In those cases, you can combine mirroring with striping to deliver
high availability and increased throughput.
You can create a mirrored-stripe volume. Stripe half of the available disks to form
one striped data plex, and stripe the remaining disks to form the other striped
data plex in the mirror. This approach is often the best way to configure a set of
disks for optimal performance with reasonable reliability. However, if a disk in
one of the plexes fails, the entire plex is unavailable.
You can also arrange equal numbers of disks into separate mirror volumes.
Afterwards, 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 43.
If a disk in a mirror fails, it does not take the disks in the other mirrors out of use.
For large volumes or large numbers of disks, a striped-mirror layout is preferred
over a mirrored-stripe layout.
RAID-5
RAID-5 offers many of the advantages of combined mirroring and striping, but
it requires more disk space. RAID-5 read performance is similar to that of striping,
and RAID-5 parity offers redundancy similar to mirroring. The disadvantages of
RAID-5 include relatively slow write performance.
RAID-5 is not usually seen as a way to improve throughput performance. The
exception is when the access patterns of applications show a high ratio of reads
to writes. .
Volume read policies
To help optimize performance for different types of volumes, VxVM lets you set
one of several read policies on data plexes.
See “Changing the read policy for mirrored volumes” on page 370.
487Performance monitoring and tuning
RAID-5