Deskstar 7K250 Hard Disk Drive Specification
89
10.10 Seek overlap
HDS7225xxVLATx0 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.
Figure 17 : Seek overlap
(1) With overlap
(2) Without overlap
Host process
Host process
Device process
Device process
Seek operation
Seek operation
Overhead
Overhead
A
A B+A
B
B
Total time = (n-1) * (Seek operation) + A + B
Total time = n * (Seek o
eration + A + B)
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