Combining Mirroring and Striping

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)). 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

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.

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.

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

Click the thumbnail above to view full-sized image.