Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!ncar!ico!ism780c!haddock!news
From: news@haddock.ima.isc.com (overhead)
Newsgroups: comp.arch
Subject: Re: Slow SCSI
Keywords: Peripheral Controllers, Gather, Scatter, I/O Architecture
Message-ID: <15249@haddock.ima.isc.com>
Date: 22 Nov 89 18:09:47 GMT
References: <35985@ames.arc.nasa.gov> <18292@watdragon.waterloo.edu> <48398@leo.UUCP>
Reply-To: suitti@anchovy.UUCP (Stephen Uitti)
Organization: Interactive Systems Co
Lines: 53

In article <48398@leo.UUCP> cgn@leo.UUCP (Chris Nieves) writes:
>The current drives being shipped by HP, Maxtor, Imprimis, etc, in the 700+MB
>range all have about the same performance coming off the disk:
>[discussion of speeds (with real data) & system performance deleted.]
>...Give SCSI a break, its only
>starting to be used in high performance applications.  Remember SCSI stands
>for Small Computer System Interface.

If you're buying new stuff, you can buy whatever is on the
market.  If you have (for example) a Mac, it might be much
cheaper to go SCSI (since you don't need a controller).  On the
other hand, I've seen SCSI systems perform at similar speeds as
ESDI systems with similar bus, CPU, and OS at the high end
(386's).  We were getting 300 Kb/sec transfers (similar to a VAX
780 with RA 81's running 4.3 BSD).

>In article <18292@watdragon.waterloo.edu>, ccplumb@rose.waterloo.edu (Colin Plumb) writes:
>> Arrays of cheap disks is one nice idea, like the CM's data vault.
>> Want 100 MB/sec?  Get 100 SCSI drives and run them in parallel.
>
>Sounds great! But what do I do when one of these inexpensive drives breaks
>and how do I go about backing up 100 SCSI drives (~70MB of data)?

Let's say you have 32 drives providing data.  Some number of
additional drives run in parallel for ECC correction.  There
might be 48 drives total, providing 2 bit correction.  Thus for
any 32 bit word - 2 bits could be wrong and yet still be reliably
computable from the total data.  Thus, you could have head
crashes on two drives and still recover all data.  You just
remove the two drives, put in two new formatted drives, tell the
CM to rebuild the two drives.

32 760 MB drives provides 24 Gigabytes of data.  There are 2+
Gigabyte helical scan (video tape) backup devices on the market.
A dozen tapes could be used to back up the entire system.  The
tapes and drives are relatively cheap.  If you are spending a
million dollars on the rest of the system, having ten tape drives
will not be an unreasonable percentage of the total cost.  I
recall dimly that the data vault can be accessed without going
through the CM, so you don't have to tie up the CM for backup.

The CM that i saw used fairly small drives - under 100 MB each.
They indicated that the system was able to use larger drives, but
of course, the smaller drives were cheaper.  If they had needed
the space, they would have spent the money.  I don't recall which
interface the CM uses.  It might not have been SCSI.

Many large shops (insurance companies) have huge disk farms.
I wonder if these shops have started moving towards the newer
low cost media.

Stephen.
suitti@haddock.ima.isc.com