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9.4
B
M
ACKGROUND
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 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 1ms.
9.5
D
A
EFERRED
UTO
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.
9.6
I
R
A
DLE
EAD
FTER
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.
S
E
P
EAGATE
NTERPRISE
ERFORMANCE
S
EDIA
CAN
-R
EALLOCATION
W
RITE
10K HDD
7 P
V
RODUCT
M
, R
. C
ANUAL
EV
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