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 | http://candle.pha.pa.us
pgman@candle.pha.pa.us | (610) 359-1001
+ If your life is a hard drive, | 13 Roberts Road
+ Christ can be your backup. | Newtown Square, Pennsylvania 19073
---------------------------(end of broadcast)---------------------------
TIP 9: the planner will ignore your desire to choose an index scan if your
joining column's datatypes do not match
---------------------------(end of broadcast)---------------------------
TIP 8: explain analyze is your friend
