3Ware 7000-2 - Escalade RAID Controller User Manual page 27

Introduction
Background Media Scan can provide early warning of a disk drive
problem or failure. For example, if the media scan encounters many
error locations, this may be an indication of excessive grown
defects on the drive. For redundant arrays, grown defects can be
fixed early to maintain optimal redundancy for the array.
Many sectors on a drive may not be used or checked for long peri-
ods of time. Bad sectors may not be known until actual user data is
written and then read from these locations. By periodic scanning of
the media, the disk drive firmware is allowed to do corrective
actions on problem areas on the disk and can minimize the occur-
rence of uncorrectable read errors.
Background Media Scan is scheduled through 3DM. When the
media scan is activated, the controller firmware either starts the
scan at the first drive with the lowest port number or scans from
when it was last deactivated. While scanning, the controller issues
normal read commands to the drive in a sequential manner. When
the controller is idle with no host commands, the rate of the media
scan is as fast as the drive can transfer data. When an error from the
drive is encountered, the controller typically will retry the com-
mand. If there are cable CRC errors, there may be multiple retries
including downgrade of the UDMA mode. If the error persists and
is repairable (e.g., ECC errors), an error notification is issued to
indicate the problem. (See "AEN 026h
AEN_DRIVE_ECC_ERROR" on page 184.) If the disk drive is
part of a redundant array that is in a redundant state (not
DEGRADED, REBUILDING), then Dynamic Sector Repair auto-
matically rewrites the redundant data to the error location to force
the drive to reallocate the error location. A notification of repair is
posted. The result is a restoration of drive and data integrity; the
primary and redundant data are again both valid.
After repairs, Background Media Scan continues on the same disk
drive until reaching the maximum logical block address on the
drive. It then proceeds onto the next disk drive available for scan-
www.3ware.com 27