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From: cgn@leo.UUCP (Chris Nieves)
Newsgroups: comp.arch
Subject: Re: Slow SCSI
Summary: SCSI speed
Keywords: Peripheral Controllers, Gather, Scatter, I/O Architecture
Message-ID: <48398@leo.UUCP>
Date: 21 Nov 89 16:43:35 GMT
References: <35985@ames.arc.nasa.gov> <18292@watdragon.waterloo.edu>
Organization: ICL North America, Irvine, CA.
Lines: 46

In article <35985@ames.arc.nasa.gov> lamaster@ames.arc.nasa.gov (Hugh LaMaster) writes:
> 1) Why are SCSI disk subsystems *so* slow.  On sequential reads, ~300 KB/sec.
> 2) Is the reason lack of gather/scatter on (inexpensive) SCSI controllers?
> 3) Are there implementation limitations of the new RISC-based systems which
> make I/O cost more than on older systems?  (e.g. VAX or 68K).  Is there
> something about SCSI in particular that is a problem?
> 5) (Not really a comp.arch question, but related to the above - aside:
> Does anyone make synchronous SCSI disks which really perform?)

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:

                 seek - track to track    ~3-4 ms
                 seek - average            ~16 ms
                 rotation                 3600 rpms   (HP 4000rpms)
                 rotational latency       8.33 ms     (HP 7.47 ms)
                 data xfer rate off disk  ~15.5 mbit/sec  =  ~1.4 mbytes/sec
                 scsi bus transfer rate   4 mbytes/sec (sync burst)

Since the controller can get data at a maximum rate of 1.4 mbytes, the
controller cannot transfer on the scsi bus at 4MB without playing tricks.
Some of the tricks are 64K buffers, continueing reading data from the disk
after the current read data is in the controllers buffer (just in case you're
doing a sequential read), and disconnecting until most of the data is in the
buffer.

It seems the high performance HDAs are getting cheaper and SCSI is getting
higher performance.  With drive companies announcing 1+ GB drives with
SCSI interfaces, you'll see better performance.  Give SCSI a break, its only
starting to be used in high performance applications.  Remember SCSI stands
for Small Computer System Interface.

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)?


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Chris Nieves
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