Seek Overlap; Figure 85. Seek Overlap - Hitachi ic35l060avv207-0 - Deskstar 60 GB Hard Drive Specifications

9.9 Seek Overlap

The Deskstar 180GXP provides an accurate method for measuring seek time. The seek command is
usually used to measure the device seek time by accumulating the execution time for a number of seek
commands. With typical implementation of 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 is completed before the actual seek operation is ended. Then the device can
receive the next seek command from the host; however, the actual seek operation for the next seek
command starts immediately after the actual seek operation for the first seek command is completed. In
other words, the execution of two seek commands overlaps excluding the time required for the actual
seek operation.
With this overlap the total elapsed time for a number of seek commands results in the total accumulated
time for actual seek operation plus one pre- and post-overhead. When the number of seeks is large, only
one overhead may be ignored.
(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)

Figure 85. Seek overlap

A
A
Deskstar 180GXP hard disk drive specifications
B+A
88
B
B