Mylex DAC960PG, Installation Manual

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Drive Management
2-20 DAC960PG Installation Guide
Critical
A logical unit is considered critical when any failure of another of its
physical drives may result in a loss of data.
A logical unit is critical if it meets both of the following conditions:
1. It is configured for RAID 1, RAID 3, RAID 5 or RAID 0+1
2. One (and no more than one) of its physical drives is not on-line
(refer to the description of Off-line, below.
Off-line
An off-line logical unit is one on which no data can be read or written. No
operations can be performed on off-line logical units. System commands
issued to off-line logical units are returned with an error status.
1. A logical unit can be off-line under one of two conditions:
2. It is configured with a redundant RAID level (1, 5, or 0+1) and two or
more of its SCSI drives are not on-line
3. It is configured as RAID 0 or JBOD (or in a spanned set) and one or
more of its SCSI drives are not on-line.
Controlling Standby Replacement Drives (Hot Spares)
The standby replacement drive, or hot spare, is one of the most important
features the DAC960PG provides to achieve automatic, non-stop service
with a high degree of fault-tolerance. With the standby rebuild function, the
controller performs a rebuild operation automatically when a SCSI disk
drive fails and both of the following conditions are true:
1. A standby SCSI disk drive of identical or larger size is found attached
to the same controller;
2. All of the system drives that are dependent on the failed disk are
redundant system drives, e.g., RAID 1, RAID 3, RAID 5, or
RAID 0+1.
During the automatic rebuild process, system activity continues as normal.
System performance may degrade slightly, however, during a rebuild.