IBM DJSA-210 Hard Drive Specifications - Page 108

Seek Overlap, The execution of two seek

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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 A Total time = (n-1) * (Seek operation) + A + B (2) Without overlap Host process Device process Seek operation Overhead A B+A Total time = n * (Seek operation + A + B) B B+A B Figure 68. Seek overlap Travelstar 32GH/30GT/20GN hard disk drive specifications 94

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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
(2) Without overlap
Host process
Host process
Device process
Device process
Seek operation
Seek operation
Overhead
Overhead
A
A
B+A
B
B+A
B
Total time = (n-1) * (Seek operation) + A + B
Total time = n * (Seek operation + A + B)
Figure 68. Seek overlap
Travelstar 32GH/30GT/20GN hard disk drive specifications
94