A mirror can consist of up to three (3) submirrors. (Practically, creating a two-way mirror is usually sufficient. A third submirror enables you to make online backups without losing data redundancy while one submirror is offline for the backup.)
If you take a submirror "offline," the mirror stops reading and writing to the submirror. At this point, you could access the submirror itself, for example, to perform a backup. However, the submirror is in a read-only state. While a submirror is offline, DiskSuite keeps track of all writes to the mirror. When the submirror is brought back online, only the portions of the mirror that were written (resync regions) are resynced. Submirrors can also be taken offline to troubleshoot or repair physical devices which have errors.
Submirrors have names like other metadevices (d0, d1, and so forth). For more information on metadevice naming, see Table 1-4.
Submirrors can be attached or detached from a mirror at any time. To do so, at least one submirror must remain attached at all times. You can force a submirror to be detached using the -f option to the metadetach(1M) command. DiskSuite Tool always "forces" a mirror detach, so there is no extra option. Normally, you create a mirror with only a single submirror. Then you attach a second submirror after creating the mirror.