SCSI and 386/ix

[Karl Denninger]

In article <PCG.90Feb5190711 at rupert.cs.aber.ac.uk> pcg at rupert.cs.aber.ac.uk (Piercarlo Grandi) writes:
>In article <1990Feb3.191842.1991 at ddsw1.MCS.COM> karl at ddsw1.MCS.COM (Karl Denninger) writes:
>
>	[ ... about having dual WD1003 style controllers ... ]
>
>   Even with ONE controller, it's not bad.  ISC >will< overlap seeks to the two
>   devices, IF your controller can handle it.  If not, you have to turn off
>   that feature in the space.c file for the device (it's on by default); this
>   is the cause of some problmes with certain revisions of the WD1006 boards.
>
>This perplexes me. You sure that a WD1003 style controller can
>overlap seeks? Are there any that do? I can hardly imagine seeks
>not being synchronous to the controller with a WD1003 style
>controller, that must issue step pulses to the head.
>
>Are there really any RLL or MFM controllers (the WD1006 is by
>your implication) that can step independently the two arms?

Sure there are!  Even the old WD1003 controllers can do this.

MOST 1006's can.  Some can't -- that is the cause of the mysterious "strange
HD errors" with 2 drives under ISC.  If you get those, have the controller
swapped or make the change I posted a while back to the space.c file for the
driver; it will go away.

The boards do this using the buffered seek feature; they don't >really<
overlap seeks per-se (since they only have one control channel that would be
difficult!); what is done is the controller sends the seek pulses rapidly
(far faster than the drive can complete them), then goes and does the other
drive.  When the drives reach their destination track(s) they report back
seek complete, and the I/O operation (one at a time) actually takes place.

This is one reason (among others) why ST412 drives are needed on an AT,
rather than ST506.  ST412 drives implement this buffered seek, while ST506
drives do not (that is pretty much the only difference between them; the
ST412 drives also support > 8 heads, which some people find important :-).

SCSI, on the other hand, can do even better.  The Adaptec host adapter is
faster than the drives; thus, it can (with a buffered drive) actually drive
two disks at 2X the speed of one.  With an ST412 controller you usually
don't get better transfer rates (total) from two drives than one; with SCSI 
it happens all the time.

--
Karl Denninger (karl at ddsw1.MCS.COM, <well-connected>!ddsw1!karl)
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