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 resync takes place, the mirror remains readable and writable by users.
A mirror resync ensures proper mirror operation by maintaining all submirrors with identical data, with the exception of writes in progress.
A mirror resync is mandatory, and cannot be omitted. You do not need to manually initiate a mirror resync; it 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 resync 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 resync is in progress, the resync is started when the system reboots and comes back up.
During a reboot following a system failure, or when a submirror that was offline is brought back online, DiskSuite performs an optimized mirror resync. The metadisk driver tracks submirror regions and knows which submirror regions may be out-of-sync after a failure. An optimized mirror resync is performed only on the out-of-sync regions. You can specify the order in which mirrors are resynced during reboot, and you can omit a mirror resync by setting submirror pass numbers to 0 (zero). (See "Pass Number" for information.)
Following a replacement of a slice within a submirror, DiskSuite performs a partial mirror resync of data. DiskSuite 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 resynced during a system reboot. The default pass number is one (1). Smaller pass numbers are resynced first. If 0 is used, the mirror resync is skipped. A 0 should be used only for mirrors mounted as read-only. Mirrors with the same pass number are resynced at the same time.