Seek Overlap; Figure 74. Seek Overlap - IBM IC35L020 - Deskstar 20 GB Hard Drive Specifications

13.10 Seek Overlap

Each drive model provides an accurate seek time measurement method. The seek command is usually
used to measure the device seek time by accumulating the execution time for a number of seek
commands. With the 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 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 the 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 74. Seek overlap

Deskstar 60 GXP Hard disk drive specification
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