Overview of Submirrors

The RAID 0 volumes that are mirrored are called submirrors. A mirror is made of one or more RAID 0 volumes (stripes or concatenations).

A mirror can consist of up to four submirrors. Practically, 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, Solaris Volume Manager keeps track of all writes to the mirror. When the submirror is brought back online, only the portions of the mirror that were written while the submirror was offline (resynchronization regions) are resynchronized. Submirrors can also be taken offline to troubleshoot or repair physical devices which have errors.

Submirrors can be attached or detached from a mirror at any time, though at least one submirror must remain attached at all times.

Normally, you create a mirror with only a single submirror. Then, you attach a second submirror after you create the mirror.