RAID 1 volume (mirror) resynchronization is the process of copying data from one submirror to another after submirror failures, system crashes, when a submirror has been taken offline and brought back online, or after the addition of a new submirror.
While the resynchronization takes place, the mirror remains readable and writable by users.
A mirror resynchronization ensures proper mirror operation by maintaining all submirrors with identical data, with the exception of writes in progress.
A mirror resynchronization is mandatory, and cannot be omitted. You do not need to manually initiate a mirror resynchronization. This process occurs automatically.
When a new submirror is attached (added) to a mirror, all the data from another submirror in the mirror is automatically written to the newly attached submirror. Once the mirror resynchronization is done, the new submirror is readable. A submirror remains attached to a mirror until it is explicitly detached.
If the system crashes while a resynchronization is in progress, the resynchronization is restarted when the system finishes rebooting.
During a reboot following a system failure, or when a submirror that was offline is brought back online, Solaris Volume Manager performs an optimized mirror resynchronization. The metadisk driver tracks submirror regions and knows which submirror regions might be out-of-sync after a failure. An optimized mirror resynchronization is performed only on the out-of-sync regions. You can specify the order in which mirrors are resynchronized during reboot, and you can omit a mirror resynchronization by setting submirror pass numbers to 0 (zero). (See Pass Number for information.)
Following a replacement of a slice within a submirror, Solaris Volume Manager performs a partial mirror resynchronization of data. Solaris Volume Manager copies the data from the remaining good slices of another submirror to the replaced slice.
The pass number, a number in the range 0–9, determines the order in which a particular mirror is resynchronized during a system reboot. The default pass number is 1. Smaller pass numbers are resynchronized first. If 0 is used, the mirror resynchronization is skipped. A pass number of 0 should be used only for mirrors that are mounted as read-only. Mirrors with the same pass number are resynchronized at the same time.