VERITAS Volume Manager 4.1 Administrator's Guide

Performance Monitoring and Tuning
Performance Guidelines
Chapter 12 397
In some cases, you can also use mirroring to improve I/O performance.
Unlike striping, the performance gain depends on the ratio of reads to
writes in the disk accesses. If the system workload is primarily
write-intensive (for example, greater than 30 percent writes), mirroring
can result in reduced performance.
Combining Mirroring and Striping
NOTE You may need an additional license to use this feature.
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 27). 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.