Seek Overlap; Table 70. Seek Overlap - Hitachi HDS725050KLA360 Specifications

10.9 Seek overlap

HDS7250xxKLAT80 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.
Table 71: Seek overlap
( 1) W i t h over l ap
Host pr ocess
Devi ce pr ocess
Seek oper at i on
Over head
Tot al t i m e = ( n- 1) * ( Seek oper at i on) + A + B
( 2) W i t hout over l ap
Host pr ocess
Devi ce pr ocess
Seek oper at i on
Over head
Tot al t i m e = n * ( Seek oper at i on + A + B)
A
A
Deskstar 7K500 and Deskstar E7K500 Hard Disk Drive Specification
B+A
90
B
B