unix, PCs and SCSI

neese at adaptex.UUCP neese at adaptex.UUCP
Wed Nov 15 01:48:00 AEST 1989

>	Now I'm wondering what to get.  I've found out that scsi tends to get
>quite expensive and I'm wondering if I really need it.  Why is scsi so much
>better then mfm and rll under unix?  I've been told that the Adaptec 1542 is a
>good controller. I've been told the Future Domain tmc860 is a good controller. 
>Where can I get these controllers and how much should I expect to pay for

You can get the AHA-1540A/1542A from Anthem, Wyle Labs, Hamilton Avnet, or
several OEM's.  I have heard street prices of $279.00.

>What sort of drive should I match these controllers to?

In the 80MB capacity range, I recommend the Quantum PRO80S.  I beleive you
can get that from Arrow.

>How much for the drives (80 meg)?

I don't know what the street prices are for the drive.

>Why not go back to rll?

SCSI has several advantages in a UNIX system.  The right combination of
hardware/drive/OS will yeild several advantages.  For instance, the 1540/1542
host adapters have scatter/gather in the hardware.  This has proven to be a
big win in a demand paging OS.  It does require the OS to support it.  ISC
has added this support into the 2.0.2 SCSI driver for the 1540/1542.  Plus the
fact that the board is a bus master (it moves the data for the CPU).  This
reduces the interrupt overhead and the bus overhead.  SCSI also offers the
advantage of multiple commands being issued to any device on the bus that is
not active (multi-threading).  As your system grows and you want to add another
drive, then your performance will increase significantly.  In an ideal world
where bothe devices are being accessed your throughput will double.

>Will it be cheaper?

My philosophy on the cost factor:  If cost is your primary concern, then
SCSI is a poor choice.  You can get cheap SCSI implementations, but then the
performance will be worse than a good RLL controller/drive combination.  If
you are more interested in performance with cost being secondary, then a good
SCSI combination (which won't much more than a comparable ESDI combo) is a
better choice.

>Will it be slower?

SCSI can be a slow as you want, or as fast as you want.  It really depends
on how far you want to reach into your pockets.  If you buy a cheap host
adapter and a cheap drive, you have completely defeated any advantage of
going SCSI.  Or if you buy a good host adapter and a cheap drive you have
wasted money on the adapter.  The flexibility of SCSI is also its major
drawback.  The interface allows drive manufacturers to build cheap slow
performing drives as well as high performance screamers.  The same applies
to the host adapter.

>Will it work? 

I can only speak for the 1540/1542 host adapters, but they do work.  Support
for these adapters is included in SCO and ISC.

I am working on an article that I will post to the net soon, that will
further describe SCSI and the things you should watch out for in buying
a SCSI implementation.

			Roy Neese
			Adaptec Central Field Applications Engineer
			UUCP @ {texbell,attctc}!cpe!adaptex!neese

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