User Manual

Constellation.2 SAS Product Manual, Rev. K 51
9.4 Background Media Scan
Background Media Scan (BMS) is a self-initiated media scan. BMS is defined in the T10 document SPC-4 available from
the T10 committee. BMS performs sequential reads across the entire pack of the media while the drive is idle. In RAID
arrays, BMS allows hot spare drives to be scanned for defects prior to being put into service by the host system. On
regular duty drives, if the host system makes use of the BMS Log Page, it can avoid placing data in suspect locations on
the media. Unreadable and recovered error sites will be logged or reallocated per ARRE/AWRE settings.
With BMS, the host system can consume less power and system overhead by only checking BMS status and results
rather than tying up the bus and consuming power in the process of host-initiated media scanning activity.
Since the background scan functions are only done during idle periods, BMS causes a negligible impact to system per-
formance. The first BMS scan for a newly manufactured drive is performed as quickly as possible to verify the media
and protect data by setting the “Start time after idle” to 5ms, all subsequent scans begin after 500ms of idle time.
Other features that normally use idle time to function will function normally because BMS functions for limited time
bursts then suspends activity to allow other background functions to operate.
BMS interrupts immediately to service host commands from the interface bus while performing reads. BMS will com-
plete any BMS-initiated error recovery prior to returning to service host-initiated commands. Overhead associated
with a return to host-servicing activity from BMS only impacts the first command that interrupted BMS, this results in a
typical delay of about 1 ms.
9.5 Media Pre-Scan
Media Pre-Scan is a feature that allows the drive to repair media errors that would otherwise have been found by the
host system during critical data accesses early in the drive’s life. The default setting for Media Pre-Scan is enabled on
standard products. Media Pre-Scan checks each write command to determine if the destination LBAs have been
scanned by BMS. If the LBAs have been verified, the drive proceeds with the normal write command. If the LBAs have
not been verified by BMS, Pre-Scan will convert the write to a write verify to certify that the data was properly written
to the disk.
Note. During Pre-Scan write verify commands, write performance may decrease by 50% until Pre-Scan com-
pletes. Write performance testing should be performed after Pre-Scan is complete. This may be checked
by reading the BMS status.
To expedite the scan of the full pack and subsequently exit from the Pre-Scan period, BMS will begin scanning imme-
diately when the drive goes to idle during the Pre-Scan period. In the event that the drive is in a high transaction traf-
fic environment and is unable to complete a BMS scan within 24 power on hours BMS will disable Pre-Scan to restore
full performance to the system.