Deferred Auto -Reallocation; Idle Read After Write; Protection Information (Pi); Levels Of Pi - Seagate ST900MM0006 Product Manual

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.
9.7 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.
9.7.1

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.
Type 3 - Seagate products do not support Type 3.
S 10K.6 SAS P
AVVIO RODUCT
-R
EALLOCATION
W
RITE
(PI)
M , R . C
ANUAL EV
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