Recoverable Errors; Unrecoverable Errors; Automatic Defect Retirement; Error Recovery Process - Western Digital Re EX800M Technical Reference Manual

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Table Of Contents

Table of Contents

Reliability

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.

4.3.5

When a sector is recovered by firmware it is marked as needing repair. When a new host command

writes to that sector, a sector test is performed by writing and reading to that location several times. If

recovery is required to read the sector during the sector test, it is relocated.

4.3.6

If an unrecoverable error is found during the offline scan, the sector is marked. Future reads from this

location will continue to perform full error recovery. However, the next write to this location will

perform a sector test to be sure the media is not damaged, and the sector relocated if the sector test fails.

4.3.7

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 are 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.

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.

4.3.8

The drive has five means of error recovery:

ECC On-the-Fly

Read/Write Retry Procedure

Extended Read Retry Procedure

ECC On-the-Fly – If an LDPC 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.

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 retry procedure applies only to read data

recovery. The Read/Write Retry procedure performs the actual retry operation.

When an extended retry operation is successful, the controller continues with the command. The

controller clears any changes in track offset or data DAC settings before the command continues.