Defect Management; Automatic Defect Retirement; Error Recovery Process - Western Digital Purple PR1200M Technical Reference Manual

WD Purple™ PR1200M

3.14.8 Defect Management

Every WD drive undergoes factory-level intelligent burn in, which thoroughly tests for
and maps out defective sectors on the media before the drive leaves the
manufacturing facility. Following the factory tests, a primary defect list is created. The
list contains the cylinder, head, and sector numbers for all defects.
Defects managed at the factory are sector slipped. Grown defects that can occur in
the field are mapped out by relocation to spare sectors on the inner cylinders of the
drive.

3.15 Automatic Defect Retirement

The automatic defect retirement feature automatically maps out defective sectors
while reading or writing. If a defective sector appears, the drive finds a spare sector.
The following item is specific to automatic defect retirement on writes (write auto-
relocation):
Data is always written to disk (using automatic defect retirement if required) and

no error is reported.
The following item is specific to automatic defect retirement on reads (read auto-
relocation):
When host retries are enabled, the drive will internally flag any unrecoverable

errors (DAMNF or ECC). This flagging allows subsequent write commands to this
location to relocate the sector only if the sector test fails.

3.15.1 Error Recovery Process

The drive has five means of error recovery:
ECC On-the-Fly

Preamp Thermal Asperity (TA) Compensation

Read/Write Retry Procedure

Extended Read Retry Procedure

ECC On-the-Fly – If an ECC error occurs, the drive attempts to correct it on-the-fly
without retries. Data can be corrected in this manner without performance penalty.
The details of the correction algorithm appear in the next section.
Preamp Thermal Asperity Compensation – A Thermal Asperity (TA) is a baseline shift in
the readback signal due to heating of the magnetoresistive stripe on the head as a
result of physical contact with the disk or a particle. The preamp circuit has the ability
to detect and compensate for thermal asperities. When an error cannot be corrected
by ECC On-the-Fly, another retry is performed, where the preamp with its thermal
asperity detection feature determines if the error is due to a thermal asperity. Once
the preamp determines that the error is due to thermal asperity, preamp
compensation is enabled. If preamp compensation alone is not enough to recover,
then the channel performs a series of TA-specific recoveries.
Read/Write Retry Procedure – This retry procedure is used by all disk controller error
types. If the procedure succeeds in reading or writing the sector being tried, then
recovery is complete and the controller continues with the command. Each retry
operation also checks for servo errors. The procedure ends when error recovery is
achieved or when all possible retries have been attempted.
Extended Read Retry Procedure – This retry procedure tries combinations of
positive/negative track offsets and data DAC manipulations to recover the data. This
2679-800065-A03 RELEASED 5/4/17 (WD CONFIDENTIAL)
Product Features
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