Path: utzoo!attcan!uunet!husc6!bloom-beacon!mit-eddie!killer!texbell!merch!cpe!neese
From: neese@cpe.UUCP
Newsgroups: comp.unix.xenix
Subject: Re: Future Domain SCSI controller for A
Message-ID: <6800054@cpe>
Date: 18 Oct 88 14:19:00 GMT
References: <522@m3.mfci.UUCP>
Lines: 41
Nf-ID: #R:m3.mfci.UUCP:522:cpe:6800054:000:2150
Nf-From: cpe.UUCP!neese    Oct 18 09:19:00 1988


>>Perhaps I was mistaken about 2.3 including Adaptec support.  My info was
>>second hand.
>
>XENIX 386 2.3 has several different versions, not all of which are out yet.
>There will be a version released which supports the Adaptec card for
>boot and root devices.
>
>>The slowness may be due to the [Future Domain] controller or the driver
>>or both.  In truth,
>>I don't care; it's just too slow.  I'm ditching Future Domain as soon as
>>possible.
>
>This is good and useful information.  It's also in direct contradiction
>to Future Domain's literature, which makes the point that PIO can often
>be faster than DMA on a 386AT architecture due to its ability to handle
>arbitrary block transfers into virtual memory versus breaking up a
>request into 4kb physical DMA transfers.  I'd be interested to know
>what "too slow" is here.  How does it compare to your experience
>with ST506 controllers under XENIX?

Let's make some general purpose statements.  Dumb SCSI adapters are akin to
dumb serial ports.  Intelligent SCSI adapters do for disk I/O what smart
serial adapters do for serial I/O.  Offload the CPU.  The AHA-1540 offloads
the SCSI low-level functions.  The CPU doesn't deal with data transfers,
bus arbitration, phase changes, error handling, handshakes,  and multiple
interrupts.  Translates to low CPU overhead.  A dream for the Unix/Xenix
system.  The only limitation of performance is the quality of the device
driver and the speed of the CPU.  The true measure of disk performance
for Unix systems is the overhead associated with disk I/O.  The lower
disk overhead, the better overall performance for the system.  The only
benefit an ESDI interface gains over ST-506 is a higher data rate and
usually better seek times.  In standard implementations, ESDI and ST-506
have almost the same disk overhead for the CPU.  We tested this theory
by running a program that mixed calculations with disk I/O and a 16Mhz
386 with a AHA-1540 SCSI adapter ran up to 53% faster than a 20Mhz 386
with an ESDI drive.  Seek times were eqivulent in both drives.

					Roy Neese
					Tandy Computer Product Engineering
					UUCP@ killer!ninja!cpe!neese