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SEAGATE ENTERPRISE CAPACITY 3.5 HDD V4 SAS PRODUCT MANUAL, REV. A 42
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 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 bursts of 800ms and then suspends activity for 100ms 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.
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.
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 disable Pre-Scan to restore full performance to the system.
9.6 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 operations performed by the drive. When a write
command is received for an LBA marked for DAR, the auto-reallocation 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 unreadable 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.
9.7 IDLE READ AFTER WRITE
Idle Read After Write (IRAW) utilizes idle time to verify the integrity of recently written data. During idle periods, no active system requests,
the drive reads recently written data from the media and compares it to valid write command data resident in the drives data buffer. Any
sectors that fail the comparison result in the invocation of a rewrite and auto-reallocation process. The process 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.
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.