ESDI vs SCSI

[Mike McNally]

In article <98520 at sun.Eng.Sun.COM> williamt at sun.UUCP (William A. Turnbow) writes:
>	Don't know if anyone had followed this up, but my experience with
>ESDI/SCSI drives leads me to believe that ESDI drives are faster.
>
>	First off you have limiting factors in bus transfer speed.  On an
>AT bus I believe this is something like around 4-5 Meg/sec.  

Try 0.5 Mbytes/sec, not 5.  The DMA controller on the AT takes a long
time getting on and off the bus.  At least one company makes a SCSI
controller for the AT that includes its own DMA controller that does
burst transfers and thus runs closer to the speeds you mentioned.

>Neither of
>these drives comes close.  The limiting factor from what I have seen is
>interleave, # sectors/track and (rarely) spin rate.  The spin rate is
>almost always 3600 RPM or 60 RPS.  Using this one can quickly see the
>maximum sustained xfer rate for a drive with 17 or 34 sectors per track
>(512 bytes/sec * # secs/trac * 60 RPS)  = 522K/sec max for standard MFM
>and 1044K/sec max for standard ESDI.

The CDC Wren V drives we've used here can (with 64K requests) sustain about
1.2 Mbytes/sec.  That's on a 20 Mhz. 386 AT.  The ESDI drives with WD
controllers only do about 600K/sec.   Intelligent SCSI drives support a
variable number of sectors per track, so one gets more out of the drive.
They also do automatic bad block remapping.  Many have integral RAM
caches.

Another big plus with SCSI is the ability to connect up to seven devices
to one controller.  Granted, this might be sort-of insane, but it can
be done.  One can go out and buy a SCSI cassette streamer for well 
under $1000 and backup 150meg of disk on one cassette.  One can also
get 9-track full size reel-to-reel SCSI drives for about $4000.


-- 
Mike McNally                                    Lynx Real-Time Systems
uucp: {voder,athsys}!lynx!m5                    phone: 408 370 2233

            Where equal mind and contest equal, go.