Mylex DAC960PG, Installation Manual

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Functional Description
Manual No. 771971 2-21
Using Standby Rebuild
To use the automatic standby rebuild feature, it is necessary to always
maintain a standby disk in the system.
A standby disk can be created when the DAC960PG configuration is created
or changed using the DACCF software utility, all disks attached to the
controller that are On-line and not assigned to a drive group will be
automatically labeled as standby disks.
Hot-Swap Drive Replacement
The DAC960PG supports the ability of certain drive enclosures to perform a
hot-swap drive replacement while the system is on-line. A disk can be
disconnected, removed, or replaced with a different disk without taking the
system off-line. The SCSI bus termination must be arranged so that a drive
can be removed without disrupting the termination scheme.
Disk Failure Detection
The DAC960PG automatically detects SCSI disk failures. A monitoring
process running on the controller checks, among other things, elapsed time
on all commands issued to disks. A time-out will cause the disk to be reset
and the command will be retried. If the command time-out occurs again, the
disk could be killed by the controller (that is, its state changed to dead).
The DAC960PG also monitors SCSI bus parity errors and other potential
problems. Any disk with too many errors will be killed by the controller.
Disk Media Error Management
The DAC960PG manages SCSI disk media errors in a manner transparent to
the user.
Disks are programmed to report errors. When a disk reports a media error
during a read, the controller reads the data from the mirror (RAID 1 or RAID
0+1), or computes the data from the other blocks (RAID 3 or RAID 5), and
writes the data back to the disk that encountered the error. If the write fails,
or the following verify-of-data fails (media error on write), the controller
issues a REASSIGN command to the disk, and then writes the data to a new
location. Since the problem has been resolved, no error is reported to the
system.