Seek Overlap; Write Cache Function; Figure 82. Seek Overlap - IBM DTLA-305040 - Deskstar 41.1 GB Hard Drive Specifications

10.9 Seek Overlap

The DTLA-3xxxxx 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 com-
mands. With typical implementation of seek command this measurement must include the device and
host command overhead. To eliminate this overhead the DTLA-3xxxxx 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 com-
mand 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.
(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

Figure 82. Seek overlap

10.10 Write cache function

Write cache is a performance enhancement whereby the device reports the completion of the write com-
mand (Write Sectors, Write Multiple, and Write DMA) to the host as soon as the device has received all
of the data into its buffer. The device assumes the responsibility of subsequently writing the data onto the
disk.
Ÿ While writing data after completed acknowledgment of a write command, soft reset or hard reset
does not affect its operation. However power off terminates the writing operation immediately and
unwritten data is lost.
Ÿ The Soft reset, Standby (Immediate) command, and Flush Cache commands during the writing of the
cached data are executed after the completion of writing to media. So the host system can confirm
the completion of write cache operation by issuing a Soft reset, Standby (Immediate) command, or
Flush Cache command and then confirming its completion. It is recommended that the host system
verify the completion of the write cache operation by issuing Soft reset, the Standby (Immediate)
command, or the Flush Cache command to the device before power off.
Ÿ The retry bit of Write Sectors is ignored when write cache is enabled.
Deskstar 40GV & 75GXP hard disk drive specifications
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