Seek Overlap; Write Cache Function; Table 59: Seek Overlap - Hitachi HTS421212H9AT00 - Travelstar 120 GB Hard Drive Specifications

11.12 Seek Overlap

The drive provides accurate seek time measurement method. The seek command is usually used to measure the
device seek time by accumulating execution time for a number of seek commands. With typical implementation of
the seek command, this measurement must include the device and host command overhead. To eliminate this
overhead, the drive overlaps the seek command as described below.
The first seek command completes before the actual seek operation is over. Then the device can receive the next
seek command from the host. However, the actual seek operation for the next seek command starts right after
completion of the actual seek operation for the first seek command. The execution of two seek commands overlaps
excluding the actual seek operation.
With this overlap, total elapsed time for a number of seek commands is the total accumulated time for the actual
seek operation plus one pre and post overhead. When the number of seeks is large, this overhead can be ignored.

Table 59: Seek overlap

(1) With overlap
Host process
Device process
Seek operation
Overhead
Total time = (n-1) * (Seek operation) + A + B
(2) Without overlap
Host process
Device process
Seek operation
Overhead
Total time = n * (Seek operation + A + B)

11.13 Write Cache function

Write cache is a performance enhancement whereby the device reports completion of the write command (Write
Sector(s) and Write Multiple) to the host as soon as the device has received all of the data in its buffer. The device
assumes responsibility to write the data subsequently onto the disk.
• 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 is lost.
• Flush cache, Soft reset, Standby, Standby Immediate, and Sleep are executed after the comple-
tion of writing to disk media on enabling write cache function. The host system can confirm
the completion of Write Cache operation by issuing Flush Cache command, Soft reset, Standby
command, Standby Immediate command, or Sleep command, and then confirming the comple-
tion of the issued command.
A
A
Travelstar 4K120 Hard Disk Drive Specification
B+A
87
B
B+A
B