Installation guide

G-2 AcceleRAID 170 Installation Guide
Disk Failure Detection
The 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 (its state changed to “offline”) could take the disk offline. Mylex
RAID controllers also monitor SCSI bus parity errors and other potential
problems. Any disk with too many errors will also be ‘offline’.
Disk Media Error Management
Mylex RAID controllers transparently manage SCSI disk media errors.
Disks are programmed to report errors, even ECC-recoverable 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. If the write 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.
Note
The soft error counter is incremented in this case. The
soft error count can be viewed using the Mylex disk
array configuration utilities.
When a disk reports a media error during a write, the controller issues a
rewrite and then, if it fails, issues a “reassign” command to the disk, and
writes the data out to a new location on the disk.
Drive Groups (or Drive Packs)
A group of individual disk drives (preferably identical) that are logically tied
to each other and are addressed as a single unit. In some cases this may be
called a drive “pack” when referring to just the physical devices. Up to 16
drives can be configured together as one drive group.
All the physical devices in a drive group should have the same size,
otherwise each of the disks in the group will effectively have the capacity of
the smallest member. The total size of the drive group will be the size of the