Sonnet Technologies Fusion RAID Configuration Tool and Utilities Operation Manual - Manual (Page 42)

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Recovery Mode
Sometimes, despite careful operation and maintenance, drives
will coincidentally fail in such a way that the RAID group
integrity is compromised. After a RAID group has been marked
offline because of problems with member drives, there is a way to
possibly recover some of the data. The guidelines and commands
listed on the following pages of this chapter can help recover data
from an offline RAID group. The following descriptions refer to
RAID 5 specifically, but the principles also extend to other RAID
types.
RAID Group Failure Scenarios
RAID groups cannot be accessed normally when their member
disks fail, and the RAID group is marked offline. RAID groups of
different RAID levels are marked offline for different reasons, as
follows:
Drive Replacement on a Failure Condition
Replace RAID Group Member Drives as Soon as They Fail
With parity and redundancy RAID levels, a RAID group can
withstand the loss of one member, and the data is still valid and
accessible. In this case, the RAID group goes into degraded mode
and uses parity or redundancy to generate the data. Although the
RAID group is fully operational, it is at risk because if any other
drive fails, data integrity is called into question.
A Warning About Drive Replacement
A very common reason that an array goes from degraded mode
to offline mode is when the wrong drive is replaced. By pulling
out a perfectly good drive, a double-drive fault occurs and there
are insufficient drives to generate data. The following procedure
is very important when you are considering removing a failed
drive, to ensure the correct drive is pulled.
Identifying Failed Drives
Prior to replacing a drive, you must be very sure which one
failed. If a failed drive is in an enclosure that supports SES (Fusion
DX800RAID, RX1600RAID, RX1600 Expansion), the drive
module's fault LED should be blinking. In that case, it is clear
which drive should be replaced. If multiple drive modules’ LEDs
are blinking, power cycling the enclosure(s) and reseating the
drives can sometimes correct intermittent conditions.
The ATTO Configuration Tool provides other methods to
identify failed drives. Please refer to Identify and Replace a Faulted
Drive on page 33 for details.
1.8 Recover Data from Offline RAID Groups
RAID Level Reason(s) for Being Marked OFFLINE Recovery Method
JBOD and RAID 1 Any drive failure See Recovery from Faults on Critical Number of Drives on
page 38
RAID 1 and RAID 10
Error during rebuild See Recovery from Failed Rebuild on page 37
Mistaken replacement of a good drive when its
mirror has failed
See Recovery from Replacement of the Wrong Drive on page 39
RAID 4 and RAID 5
Errors on two or more drives See Recovery from Faults on Critical Number of Drives on
page 38
Error during rebuild See Recovery from Failed Rebuild on page 37
Mistaken replacement of a good drive when
another member of the RAID group has failed
See Recovery from Replacement of the Wrong Drive on page 39
RAID 6
Errors on three or more drives See Recovery from Faults on Critical Number of Drives on
page 38
Error during rebuild See Recovery from Failed Rebuild on page 37
Mistaken replacement of good drive(s) when
another member of the RAID group has failed
See Recovery from Replacement of the Wrong Drive on page 39