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9.8
Write Cache Function
Write cache is a performance enhancement whereby the device reports as completing the write command (Write
Sector(s), Write Multiple and Write DMA) to the host as soon as the device has received all of the data into its buffer.
And the device assumes responsibility to write the data subsequently onto the disk.
While writing data after completed acknowledgment of a write command, soft reset or hard reset does not affect
its operation. But power off terminates writing operation immediately and unwritten data are to be lost.
Soft reset, Standby (Immediate) command and Flush Cache commands during writing the cached data are
executed after the completion of writing to media. So the host system can confirm the completion of write cache
operation by issuing Soft reset, Standby (Immediate) command or Flush Cache command to the device before
power off.
9.9
Reassign Function
The reassign Function is used with read commands and write commands. The sectors of data for reassignment are
prepared as the spare data sector.
This reassignment information is registered internally, and the information is available right after completing the
reassign function. Also the information is used on the next power on reset or hard reset.
If the number of the spare sector reaches 0 sectors, the reassign function will be disabled automatically.
The spare tracks for reassignment are located at regular intervals from Cylinder 0. As a result of reassignment, the
physical location of logically sequenced sectors will be dispersed.
9.9.1
Auto Reassign Function
The sectors those show some errors may be reallocated automatically when specific conditions are met. The spare
tracks for reallocation are located at regular intervals from Cylinder 0. The conditions for auto-reallocation are
described below.
None recovered write errors
When a write operation cannot be completed after the Error Recovery Procedure (ERP) is fully carried out, the
sector(s) are reallocated to the spare location. An error is reported to the host system only when the write cache is
disabled and the auto reallocation is failed.
If the write cache function is ENABLED, and when the number of available spare sectors reaches 0 sectors, both
auto reassign function and write cache function are disabled automatically.
None recovered read errors
When a read operation is failed after defined ERP is fully carried out, a hard error is reported to the host system. This
location is registered internally as a candidate for the reallocation. When a registered location is specified as a target
of a write operation, a sequence of media verification is performed automatically. When the result of this verification
meets the criteria, this sector is reallocated.
Recovered read errors
When a read operation for a sector failed once then recovered at the specific ERP step, this sector of data is
reallocated automatically. A media verification sequence may be run prior to the relocation according to the
pre-defined conditions.
Western Digital Hard Disk Drive OEM Specification
57
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