Newsgroups: can.usrgroup
Path: utzoo!lsuc!eci386!woods
From: woods@eci386.uucp (Greg A. Woods)
Subject: Re: Hard Drives: SCSI or ESDI
Message-ID: <1989Oct13.194516.3141@eci386.uucp>
Keywords: SCSI, ESDI, disk, interface
Reply-To: woods@eci386.UUCP (Greg A. Woods)
Organization: R. H. Lathwell Associates: Elegant Communications, Inc.
References: <891011142510.20869@tmsoft.uucp> <700@ecicrl.UUCP>
Date: Fri, 13 Oct 89 19:45:16 GMT
Lines: 42

In article <700@ecicrl.UUCP> clewis@ecicrl.UUCP (Chris Lewis) writes:
> In article <891011142510.20869@tmsoft.uucp> Tak Ariga <tak@doe.UToronto.CA> writes:
> 
> >With all this talk about hard drive interleave and capacity, I was wondering
> >whether I should go ESDI or SCSI for our next big hard drive for our Xenix
> >system.
> 
> SCSI has a few advantages over ESDI.  For example, SCSI is a peripheral
> bus, but ESDI is a *disk* bus.  Hence, you can't hang your tape drive,
> or zuper-whizzbang video interface on a ESDI interface, but you can on 
> a SCSI.  Then again, bus contention usually makes the performance pretty 
> lousy.

I'm not sure what the actual limits of the ESDI interface are, but I
know the WD1007 controller will only support 2 drives.  This give SCSI
the advantage when many drives are required.

As for performance, it can be shown that using multiple drives in a
multi-user system can be a big win, when you have a smart OS (i.e. disk
driver) and an interface such as SCSI 2 which will support multiple
asyncronous commands.  As Chris stated, current drives don't tax
either interface in the bandwidth department, though a full complement
of 8 fast devices on a SCSI buss might be pushing the limit.

> Do NOT under any circumstances use a dumb SCSI or ESDI board that
> doesn't have it's own DMA facilities.  AT DMA is ~300Kb/sec. - eg,
> *very slow*.  Further, don't use controllers that do programmed I/O.

My 3B2 SCSI host adapter has a fast 80186 onboard!  The main CPU never
waits, unless that's all it has to do.  Modern 3B2 systems often use
SCSI host adapters and "bridge" controllers such that you can hang 8
bridge controllers, each with 4 large ST506 or ESDI drives, from one
host adapter.  The 3B2 architecture allows up to 8 host adapters.
That's a lot of disk!

All in all, I find the SCSI architecture much more elegant....
-- 
						Greg A. Woods

woods@{eci386,gate,robohack,ontmoh,tmsoft,gpu.utcs.UToronto.CA,utorgpu.BITNET}
+1-416-443-1734 [h]  +1-416-595-5425 [w]		Toronto, Ontario CANADA
