Table of Contents
Advertisement
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.
During Pre-Scan write verify commands, write performance may decrease by 50% until Pre-Scan completes. Write
Note
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 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.
Seagate Exos 7E8 SAS Product Manual, Rev.A
39
Advertisement
Table of Contents
