Configuration Guidelines for RAID 1 Volumes

• When creating a RAID 1 volume from an existing file system built on a slice, only the single slice may be included in the primary RAID 0 volume (submirror). If you are mirroring root or other system-critical file systems, all submirrors must consist of only a single slice.

• 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 tends 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 brands of disk or controller can have widely varying performance. Mixing the different performance levels in a single mirror can cause performance to degrade significantly.

• Use the same size submirrors. Submirrors of different sizes result in unused disk space.

• Only mount the mirror device directly. Do not try to mount a submirror directly, unless it is offline and mounted read-only. Do not mount a slice that is part of a submirror. This process could destroy data and crash the system.

• Mirroring might improve read performance, but write performance is always degraded. Mirroring improves read performance only in threaded or asynchronous I/O situations. No performance gain results if there is only a single thread reading from the volume.

• Experimenting 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 it tends to work best for UFS multiuser, multiprocess 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, when only one process at a time is using the slice/file system, and when I/O patterns are highly sequential or when all accesses are read.

  To change mirror options, see How to Change RAID 1 Volume Options.

• Use the swap -l command to check for all swap devices. Each slice that is specified as swap must be mirrored independently from the remaining swap slices.

• Use only similarly configured submirrors within a mirror. In particular, if you create a mirror with an unlabeled submirror, you will be unable to attach any submirrors that contain disk labels.

If you have a mirrored file system in which the first submirror attached does not start on cylinder 0, all additional submirrors you attach must also not start on cylinder 0. If you attempt to attach a submirror starting on cylinder 0 to a mirror in which the original submirror does not start on cylinder 0, the following error message displays:

can't attach labeled submirror to an unlabeled mirror 

Starting cylinders do not have to be the same across all submirrors, but all submirrors must either include or not include cylinder 0.