User Manual

Constellation.2 SAS Product Manual, Rev. K 52
9.6 Deferred Auto-Reallocation
Deferred Auto-Reallocation (DAR) simplifies reallocation algorithms at the system level by allowing the drive to reallo-
cate unreadable locations on a subsequent write command. Sites are marked for DAR during read operations per-
formed 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 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 trans-
ferred 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 typ-
ically 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.