Figure 12 Seek Overlap; Write Cache Function - Hitachi HDS721050CLA362 Specifications

Seek Overlap
9.8
HDT7210xxSLA3y0 / HCT7210xxSLA3y0 / HDE7210xxSLA3y0 provides accurate seek time measurement
method. The seek command is usually 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
including the device and host command overhead. To eliminate this overhead, HDT7210xxSLA3y0 /
HCT7210xxSLA3y0 / HDE7210xxSLA3y0 overlaps the seek command as described below.
The first seek command completes before the actual seek operation is end. Then device can receive the next
seek command from the host but actual seek operation for the next seek command starts right after the actual
seek operation for the first seek command is completed. In other words, the executions of two seek commands
overlaps excluding the actual seek operation.
With this overlap, total elapsed time for a number of seek commands results the total accumulated time for
actual seek operation plus one pre and post overhead. When the number of seeks is large, this just one
overhead can be ignored.
( 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)

Write Cache Function

9.9
Write cache is a performance enhancement whereby the device reports as completing the write command
(Write Sector(s), Write Multiple and Write DMA) to the host as soon as the device has received all of the data
into its buffer. And the device assumes responsibility to write the data subsequently onto the disk.
 While writing data after completed acknowledgment of a write command, soft reset or hard reset does not
affect its operation. But power off terminates writing operation immediately and unwritten data are to be lost.
 Soft reset, Standby (Immediate) command and Flush Cache commands during writing 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 Soft reset, Standby (Immediate) command or Flush Cache command to the
device before power off.
HITACHI Deskstar & CinemaStar 7K1000.B & Deskstar E7K1000 Hard Disk Drive specification
A
A

Figure 12 Seek overlap

B+A
69
B
B