Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!wuarchive!brutus.cs.uiuc.edu!ginosko!uunet!cbmvax!jesup
From: jesup@cbmvax.UUCP (Randell Jesup)
Newsgroups: comp.arch
Subject: Re: desperately seeking explanation of typical SCSI configuration
Message-ID: <8143@cbmvax.UUCP>
Date: 11 Oct 89 04:48:11 GMT
References: <8774@spool.cs.wisc.edu> <196@ensub.Wichita.NCR.COM>
Reply-To: jesup@cbmvax.UUCP (Randell Jesup)
Organization: Commodore Technology, West Chester, PA
Lines: 42

In article <196@ensub.Wichita.NCR.COM> dsleffel@ensub.Wichita.NCR.COM (Don Sleffel) writes:
>In article <8774@spool.cs.wisc.edu> shekita@provolone.cs.wisc.edu (Eugene Shekita) writes:
>    ---------------   ------------   ----------------
>   |Disk Controller| |Tape        | |Disk Controller |
>    ---------------  |Controller  |  ----------------
>     |                ------------       |
>     |--Disk Drive     |                 |--Disk Drive
>     |                 |--Tape Drive     |
>     |--Disk Drive                       |--Disk Drive
>     |                                   |
>     etc.                                etc.

	This can be done, but is rare (use of Logical Unit Numbers).  Most
are single drive/disk controller.

>Not neccessarily.  There are a lot of different reasons why you might or
>might not want to use dual ported memory and why you might or might not
>want to use the side opposite the processor for IO operations.  Keep in mind
>there are a lot of different classes and application types of machines.

	Dual ported memory is _expensive_ (and usually not dense), as well.

>>   4) Is it becoming common for SCSI controllers to have a track-size 
>>      buffer of their own and always do track-size reads? (I am told 
>>      that Conner contollers have this feature.)
>
>Yes, as well as various forms of caching.  SCSI used on "Big Iron" will
>probably have from 2 to 200 Megabyte of buffer space, depending upon the
>type of usage.

	There are two different type of buffers: buffers in the drive/controller
and buffer in the host adaptor.  Many drives (such as Quantum) now have cache
on the controller (64K on the Q80s).  Most DMA host adapters have some form
cache or FIFO to cover main memory not being available for DMA.

	However, I've never heard of 2Meg of cache, let alone 200Meg.  If
you can afford that much memory, why buy disks?  (hyperbole, no flames please)

-- 
Randell Jesup, Keeper of AmigaDos, Commodore Engineering.
{uunet|rutgers}!cbmvax!jesup, jesup@cbmvax.cbm.commodore.com  BIX: rjesup  
Common phrase heard at Amiga Devcon '89: "It's in there!"