Good question.  If the SCSI system was moving the head from track 1 to 10, and 
a request then came in for track 5, could the system make the head stop at 
track 5 on its way to track 10?  That is something that only the controller 
could do.  However, I have no idea if SCSI does that.

||  SCSI, AFAIK, does NOT do this. What SCSI can do is allow "next" request 
insertion into head
    of request queue (queue-jumping), and/or defer request ordering to done by 
drive per se (queue
    re-ordering).   I  have looked, in vain, for evidence that SCSI somehow 
magically "stops in the
    middle of request to pick up data" (my words, not yours) 

The only part I am pretty sure about is that real-world experience shows SCSI 
is better for a mixed I/O environment.  Not sure why, exactly, but the command 
queueing obviously helps, and I am not sure what else does.

||  TCQ is the secret sauce, no doubt. I think NCQ (the SATA version of per se 
drive request reordering) 
   should go a looong way (but not all the way) toward making SATA 'enterprise 
acceptable'. Multiple 
   initiators (e.g. more than one host being able to talk to a drive) is a 
biggie, too. AFAIK only SCSI
   drives/controllers do that for now. 

  Bruce Momjian                        |               |  (610) 359-1001
  +  If your life is a hard drive,     |  13 Roberts Road
  +  Christ can be your backup.        |  Newtown Square, Pennsylvania 19073

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