Performance Guidelines for RAID-1 Volumes

• Keep the slices of different submirrors on different disks and controllers. Data protection is diminished considerably if slices of two or more submirrors of the same mirror are on the same disk. Likewise, organize submirrors across separate controllers, because controllers and associated cables tend to fail more often than disks. This practice also improves mirror performance.

• Use the same type of disks and controllers in a single mirror. Particularly in old SCSI storage devices, different models or different brands of disks or controllers can have widely varying performance. If disks and controller that have the different performance levels are used in a single mirror, performance can degrade significantly.

• Mirroring might improve read performance, but write performance is always degraded. Mirroring improves read performance only in threaded or asynchronous I/O situations. A single thread reading from the volume does not provide a performance gain.

• You can experiment with the mirror read policies can improve performance. For example, the default read mode is to alternate reads in a round-robin fashion among the disks. This policy is the default because round-robin tends to work best for UFS multiuser, multiprocessor activity.

  In some cases, the geometric read option improves performance by minimizing head motion and access time. This option is most effective when:

  • There is only one slice per disk

  • Only one process at a time is using the slice or file system

  • I/O patterns are highly sequential or when all accesses are read

• You can attach a submirror to a mirror without interrupting service. You attach submirrors to mirrors to create two-way, three-way, and four-way mirrors.

• When you place a submirror offline, you prevent the mirror from reading from and writing to the submirror. However, you preserve the submirror's logical association to the mirror. While the submirror is offline, Solaris Volume Manager keeps track of all writes to the mirror. The writes are written to the submirror when it is brought back online. By performing an optimized resynchronization, Solaris Volume Manager only has to resynchronize data that has changed, not the entire submirror. When you detach a submirror, you sever its logical association to the mirror. Typically, you place a submirror offline to perform maintenance. You detach a submirror to remove it.