RAID 1 Conventions

In general, RAID 1 volumes are much like other logical volumes. However, because of the necessity to keep two or more identical copies of all data, RAID 1 volumes must be resynchronized after new submirrors are attached, after submirrors are brought back online, or after submirror failures (with or without hot spare replacement).

Resyncronization

RAID 1 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 RAID 1 volume remains readable and writable by users.

A RAID 1 resynchronization ensures proper RAID 1 operation by maintaining all submirrors with identical data, with the exception of writes in progress.
 


Note - A RAID 1 resynchronization is mandatory, and cannot be omitted. You do not need to manually initiate a RAID 1 resynchronization; it occurs automatically.


Full RAID 1 Resynchronization

When a new submirror is attached (added) to a RAID 1 volume, all the data from another submirror in the RAID 1 volume is automatically written to the newly attached submirror. Once the RAID 1 resynchronization is done, the new submirror is readable. A submirror remains attached to a RAID 1 volume 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.

Optimized RAID 1 Resynchronization

During a reboot following a system failure, or when a submirror that was offline is brought back online, Enhanced Storage performs an optimized RAID 1 resynchronization. The metadisk driver tracks submirror regions and knows which submirror regions may be out-of-sync after a failure. An optimized RAID 1 resynchronization is performed only on the out-of-sync regions. You can specify the order in which RAID 1 volumes are resynced during reboot, and you can omit a RAID 1 resync by setting submirror pass numbers to 0 (zero). (See Pass Number for information.)
 


Caution - A pass number of 0 (zero) should only be used on RAID 1 volumes mounted as read-only.


Partial RAID 1 Resync

Following a replacement of a slice within a submirror, Enhanced Storage performs a partial RAID 1 resync of data. Enhanced Storage copies the data from the remaining good slices of another submirror to the replaced slice.

Pass Number

The pass number, a number in the range 0-9, determines the order in which a particular RAID 1 is resynced during a system reboot. The default pass number is one (1). Smaller pass numbers are resynced first. If 0 is used, the RAID 1 resync is skipped. A 0 should be used only for RAID 1 volumes mounted as read-only. RAID 1 volumes with the same pass number are resynced at the same time.