ESDI vs SCSI

[William A. Turnbow]

In article <5436 at lynx.UUCP> m5 at lynx.UUCP (Mike McNally) writes:
>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.
>
>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.
>
>Mike McNally                                    Lynx Real-Time Systems


--
1)  I said bus transfer speed.  Not DMA.  I don't know if the DMA speed
is different.

2) Can you explain how bot the ESDI and SCSI drives you mention (1.2M/s and
.6M/s) are faster than the maximum DMA rate you claim?  

3) Last time I looked at the AT bios it didn't use DMA in the disk
routines.  So it really isn't a factor.

4) You're right.  The WD ESDI controller sucks.  It seems to usually
miss tracks and give a transfer rate closer to that of a 2:1 interleave
drive than a 1:1.  The Adaptek controller I have has been measured at
slightly over 1M/s xfer rate raw, and over 800K/sec through DOS.

5) As for SCSI controllers supporting variable number of sectors per track,
that is only in the interface presented to the user (computer).  Since
the SCSI controllers have intelligence in them, they can remap the physical
sectors in whatever fashion they choose.  This does not change the
actual physical number of sectors/track, the spin rate, nor the final
maximum transfer rate.  We evaluated a couple SCSI drives at my last
job, and several of them could present 63 sectors/track to the BIOS so that
DOS could use disks that had > 1023 tracks (the BIOS doesn't support
more than 1023 tracks or greater than 63 sectors/track).  Presenting 63
sectors per track didn't physically change the media to allow greater
sector packing.  The transfer rate was the same as at the lower 
sectors per track (around 30 something...I believe).

6) Yes, it is useful to have the SCSI port if you have a need or use
for it.  But most people don't.  I had a friend who gave that same
argument, but because he had no SCSI devices to hook up, and because the
SCSI drives were uniformly slower in seek time (around 2-3 ms) (xfer
rate was about the same), and because the drives were uniformly more
expensive, he went with an ESDI.  At the time he was shopping, the SCSI
drives were running about 33% more than equivalent capacity ESDI's.

7) Be careful about how you measure disk xfer speed.  I have seen 'coretest'
be totally fooled by one controller that kept an on board cache into believing
it could xfer over 5Meg/sec on a 386.  Some tests also seem to be less
than reliable on ESDI drives because it is apparently common or standard
to include a track buffer.