From: Mike N. 

On Fri, 14 Apr 2006 09:44:04 +0300, "Antti Kurenniemi"
 wrote:

>>   There is still a benefit to having a write request queued to the target
>> drive while having a read request queued to the source drive rather than
>> waiting for the data to arrive before passing it on to the target.    Both
>> drives would always have an outstanding read or write request rather than
>> being idle while the packet is transferred to the other drive.
>
>Yes but that's what the buffer memory in the drive is for, no need to try to
>implement a second buffer in the OS level.

  Visualizing what the CopyFile routine does:

 R=read
 W=write
 I=idle
 S=Source Drive
 D = destination drive

S D
R I Start reading file 1
R I
R I   File1 completely read from source
I W Start writing file 1
I W
I W File1 completely written to target R I  Start reading File 2
R I.....

  With a single threaded model, one of the drives is completely idle for
half the time.     I don't know if CopyFile is internally multithreaded. In
the normal case when the source and target are on the same device, it might
not make sense to have it multithreaded.   When copying drive to drive it
could clearly benefit.

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