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CONSTELLATION ES.3 SAS PRODUCT MANUAL, REV. B 43
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.
9.8 PROTECTION INFORMATION (PI)
Protection Information is intended as a standardized approach to system level LRC traditionally provided by systems using 520 byte for-
matted LBAs. Drives formatted with PI information provide the same, common LBA count (i.e. same capacity point) as non-PI format-
ted 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 typi-
cally 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.
9.8.1 Levels of PI
There are 4 types of Protection Information.
Type 0 - Describes a drive that is not formatted with PI information bytes. This allows for legacy support in non-PI systems.
Type 1 - Provides support of PI protection using 10 and 16 byte commands. The RDPROTECT and WRTPROTECT bits allow for
checking control through the CDB. Eight bytes of Protection Information are transmitted at LBA boundaries across the interface if
RDPROTECT and WRTPROTECT bits are nonzero values. Type 1 does not allow the use of 32 byte commands.
Type 2 - Provides checking control and additional expected fields within the 32 byte CDBs. Eight bytes of Protection Information are
transmitted at LBA boundaries across the interface if RDPROTECT and WRTPROTECT bits are nonzero values. Type 2 does allow the
use of 10 and 16 byte commands with zero values in the RDPROTECT and WRTPROTECT fields. The drive will generate 8 bytes
(e.g.0xFFFF) 8 bytes of Protection Information to be stored on the media, but the 8 bytes will not be transferred to the host during a read
command.
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.