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9.5
M
P
-S
EDIA
RE
CAN
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.
Note.
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 disable Pre-Scan to restore full performance to the
system.
9.6
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.
9.7
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.
9.8
P
I
ROTECTION
NFORMATION
Protection Information is intended as a standardized approach to system level LRC traditionally provided by systems using
520 byte formatted LBAs. Drives formatted with PI information provide the same, common LBA count (i.e. same capacity
point) as non-PI formatted drives. Sequential performance of a PI drive will be reduced by approximately 1.56% due to the
extra overhead of PI being transferred from the media that is not calculated as part of the data transferred to the host. To
determine the full transfer rate of a PI drive, transfers should be calculated by adding the 8 extra bytes of PI to the transferred
LBA length, i.e. 512 + 8 = 520. PI formatted drives are physically formatted to 520 byte sectors that store 512 bytes of
customer data with 8 bytes of Protection Information appended to it. The advantage of PI is that the Protection Information
bits can be managed at the HBA and HBA driver level. Allowing a system that typically does not support 520 LBA formats to
integrate this level of protection.
Protection Information is valid with any supported LBA size. 512 LBA size is used here as common example.
S
10K.5 SAS P
AVVIO
RODUCT
-R
EALLOCATION
W
RITE
(PI)
M
, R
. J
ANUAL
EV
44
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