Seagate Cheetah 15K.5 SAS Series, Product Manual

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40 Cheetah 15K.5 SAS Product Manual, Rev. C
7.6 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 oper-
ations performed by the drive. When a write command is received for an LBA marked for DAR, the auto-reallo-
cation 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 unread-
able 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.
7.7 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.