Mylex Global Array Manager Installation Manual And User's Manual page 182

Glossary
Degraded Mode
A RAID mode used when a component drive has failed.
Device Driver
A software program that controls a particular type of device attached to a
computer, such as a RAID subsystem, printer, display, CD-ROM, disk drive,
etc.
Disk Failure Detection
A RAID controller 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 causes the disk to be "reset"
and the command to be retried. If the command times out again, the
controller could take the disk "offline." Mylex DAC960 controllers also
monitor SCSI bus parity errors and other potential problems. Any disk with
too many errors will also be taken "offline." See also Offline.
Disk Media Error Management
Mylex controllers transparently manage disk media errors. Disks are
programmed to report errors, even ECC-recoverable errors. If ECC RAM is
installed, the controller will correct ECC errors. When a disk reports a media
error during a read, the controller reads the data from the mirror (RAID 1 or
0+1), or computes the data from the other blocks (RAID 3, RAID 5), and
writes the data back to the disk that encountered the error. When a disk
reports a media error during a write, the controller issues a "reassign"
command to the disk, then writes the data out to a new location on the disk.
Since the problem has been resolved, no error is reported to the system.
Glossary-6 GAM Client User's Guide