Seek Overlap; Figure 68. Seek Overlap - IBM DJSA-210 - Travelstar 10 GB Hard Drive Specifications

11.11 Seek Overlap

DJSA-XXX drives provide accurate seek time measurement method. The seek command is usualy 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 DJSA-XXX 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, 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.
(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 68. Seek overlap

A
A
Travelstar 32GH/30GT/20GN hard disk drive specifications
B+A
94
B
B+A
B