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46 Constellation.2 SAS Product Manual, Rev. H
10.4 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 activ-
ity.
Since the background scan functions are only done during idle periods, BMS causes a negligible impact to sys-
tem performance. 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
complete 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.
10.5 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
completes. 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
immediately when the drive goes to idle during the Pre-Scan period. In the event that the drive is in a high
transaction traffic environment and is unable to complete a BMS scan within 24 power on hours BMS will dis-
able Pre-Scan to restore full performance to the system.
10.6 Deferred Auto-Reallocation
Deferred Auto-Reallocation (DAR) simplifies reallocation algorithms at the system level by allowing the drive to
reallocate unreadable locations on a subsequent write command. Sites are marked for DAR during read oper-
ations performed by the drive. When a write command is received for an LBA marked for DAR, the auto-reallo-
cation process is invoked and attempts to rewrite the data to the original location. If a verification of this rewrite
fails, the sector is re-mapped to a spare location.
This is in contrast to the system having to use the Reassign Command to reassign a location that was unread-
able and then generate a write command to rewrite the data. DAR is most effective when AWRE and ARRE
are enabled—this is the default setting from the Seagate factory. With AWRE and ARRE disabled DAR is
unable to reallocate the failing location and will report an error sense code indicating that a write command is
being attempted to a previously failing location.