Multiple Disks Failed - Theory - Bosch DIP-7080-00N Installation Manual

38 en | Recovering the unit
The data area can only be deleted with a "fast" or "full" initialization; as long as an initialization
is omitted, the OS file system is still there. But OS only boots, if the original RAID
configuration is restored (if no multiple hardware damage exists).
If, for example, the RAID is deleted accidently (eg. with "clear" or "new configuration" instead
of add),and the configuration is set up exactly (in disk order and stripesize) the same as
before, the data area on the remaining operating system boots again without any problem.
This fact is useful when the RAID (COD) was lost for some reason, but the disks are OK.
7.2.1 Multiple disks failed – theory
If multiple disks have failed (by power failure, backplane error, etc.), it is important to know
the order, which disk failure degraded the RAID (first fail) and which disk ID prevented a
further access to the RAID (second fail).
For instance:
RAID 5 (4 disks) got from rebuild to offline, no Hot Spare
- 2 disks online
- 2 disks missing
- 2 disks "foreign configuration" or "unconfigured good"
The rebuild only could start, when the other three disks were online before: the rebuild disk is
"first fail", degraded RAID was rebuilding. "Foreign" disk is the "second fail" disk, one of the
disks, from which the parity data was copied to the rebuilding disk before the crash.
If in "degraded" mode, the surviving disks was still used, a "parity – inconsistency" between
the first failed (first fail) and the second failed disk occurs. But a RAID 5 is not usable when a
second disk fails, so no inconsistency can exist.
If you try to restore the RAID, the first fail disk could be used for rebuild later (or a new disk).
But the "second failed" disk must be used for hopefully get the offline RAID degraded again.
Two tools are useful to analyze what happened:
- the Event Viewer in the Controller Bios Utility
- the MegaCLI, a Command-Line Utility
2014.12 | V4 | DOC Installation Manual
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