APC RAID Subsystem SCSI-SATA II - Manual (Page 68)

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Chapter 4 Advanced operation
4.1 Rebuild
If one physical disk of the VG which is set as protected RAID level (e.g.: RAID 3 ,
RAID 5, or RAID 6) is FAILED or has been unplugged/removed, then, the VG
status is changed to degraded mode, the system will search/detect spare disk to
rebuild the degraded VG to a complete one. It will detect dedicated spare disk
as rebuild disk first, then global spare disk.
IS16GL support Auto-Rebuild function. When the RAID level allows disk failures
which the VG is protected, such as RAID 3, RAID 5, RAID 6, and etc, IS16GL
starts Auto-Rebuild as below scenario:
Take RAID 6 for example:
1. When there is no global spare disk or dedicated spare disk on the system,
IS16GL will be in degraded mode and wait until (A) there is one disk assigned as
spare disk, or (B) the failed disk is removed and replaced with new clean disk,
then the Auto-Rebuild starts. The new disk will be a spare disk to the original VG
automatically.
a. If the new added disk is not clean (with other VG information), it would
be marked as RS (reserved) and the system will not start "auto-rebuild".
b. If this disk is not belonging to any existing VG, it would be FR (Free)
disk and the system will start Auto-Rebuild function.
c. if user only removes the failed disk and plugs the same failed disk in the
same slot again, the auto-rebuild will start by this case. But rebuilding in the
same failed disk may impact customer data later from the unstable disk status.
IS16GL suggests all customers not to rebuild in the same failed disk for better
data protection.
2. When there is enough global spare disk(s) or dedicated spare disk(s) for the
degraded array, IS16GL starts Auto-Rebuild immediately. And in RAID 6, if there
is another disk failure happening during the time of rebuilding, IS16GL will starts
the above Auto-Rebuild scenario as well. And the Auto-Rebuild feature only
works at "RUNTIME". It will not work the downtime. Thus, it will not conflict with
the “Roaming” function.
In degraded mode, the status of VG is “DG”.