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44 Cheetah 10K.7 SCSI Product Manual, Rev. D
7.4 Background Media Scan
Background Media Scan (BGMS) is a self-initiated media scan. It performs sequential reads across the entire
pack of the media while the drive is idle. In RAID arrays, BGMS allows hot spare drives to be scanned for
defects prior to being put into service by the host system. On regualr duty drives, if the host system makes use
of the BGMS Log Page, it can avoid placing data in suspect locations on the media. Unrecovered error sites
found during BGMS will be logged and recovered error sites will be logged or reallocated per ARRE/AWRE
settings.
With BGMS, the host system can consume less power and system overhead by only checking BGMS status/
results rather than tying up the bus and consuming power in the process of host-initiated media scanning activ-
ity.
Since the background scan functions are only done during idle periods, BGMS causes a negligible impact to
system performance. Background media scan will begin after 500ms of idle time. Other features that normally
use idle time to function will function normally because BGMS functions for bursts of 800ms and then sus-
pends activity to allow other background functions to operate. Additionally, BGMS operates in segments of 400
block reads, checking for host commands upon completion of each of the block segments.
The BGMS function will complete any BGMS-initiated error recovery prior to returning to service host-initiated
commands. Overhead associated with a return to host-servicing activity from BGMS only impacts the first com-
mand that interrupted BGMS. This results in a typical delay of about 4 ms.
7.4.1 Media Pre-Scan
Media Pre-Scan is an optional feature that allows the drive to handle unrecovered media errors that would oth-
erwise have been found by the host system during critical data accesses early in the integration process at
customer sites. This should reduce integration DPPM (defective parts per million) by identifying and repairing
media errors that arose during shipping and handling. Media Pre-Scan replaces all host-initiated Write com-
mands with Write-Verify commands when the drive is accessing areas of the media yet untouched by BGMS-
initiated scanning activity.
The drive will attempt to run BGMS on the entire pack of the drive. This will result in increasingly large portion
so fthe drive having been accesed, provided there is adequate idle time to allow BGMS to run. During the Pre-
Scan period, all Write commands that are in un-BGMS-scanned areas are converted to Write-Verify com-
mands. To expedite the scan of the full pack and the subsequenty exit from the Pre-Scan period, BGMS will
begin immediately when the drive goes to idle during the Pre-Scan period, as opposed to waiting 500ms as is
done under non-Pre-Scan conditions.