NetNews Usenet Archive 1993 #1/NN_1993_1.iso/spool/comp/sys/sgi/18680

Text File | 1993-01-11 | 3.4 KB | 70 lines

Newsgroups: comp.sys.sgi
Path: sparky!uunet!pipex!warwick!doc.ic.ac.uk!cc.ic.ac.uk!imperial.ac.uk!vulture
From: [e-mail] (Thomas Sippel - Dau)
Subject: Re: SCSI floppy disk on INDIGO: Why is it so slooow?
Message-ID: <[e-mail]>
Sender: [e-mail] (Thomas Sippel - Dau)
Nntp-Posting-Host: cscgc
Reply-To: [e-mail]
Organization: Imperial College of Science, Technology and Medicine
References: <[e-mail]> <[e-mail]> <[e-mail]>
Date: Mon, 11 Jan 93 19:42:34 GMT
Lines: 56

In article <[e-mail]>, [e-mail] (Frans van Hoesel) writes:
-
- Because mssing the next sector would be a very important time factor,
- maybe it would be nice to have a special format on the floppy that
- does not have sequential sector numbers. This would only help for
- newly formatted floppies, not for existing ones.
-
- On the other hand... knowing the effect of missing a sector *and* having
- softwarebuffers on the host a simple software trick could speed it up.
- Imagen a write of a shortfile on sector numbers 1, 2, 3, 4, 5, 6, 7.
- These could all be present in the software buffer, but we (in fact
- your engineer working on it) could decide to write in the order
- 1, 3, 5, 7, 2, 4, 6 which would give a speed up by 350% (for this very
- short write, longer writes will speed up even more)

I always knew that people diskussing perfomance problems on micro computers
would insist of re-experiencing the problems that had long since been solved
on mainframes, and object to even listening to the solutions found there
because "they do not apply to micros".

It is kind of a new experience to see workstation users display the same
block-headedness.

Yes, you can implement interleaving, sector prefetching and sector out
of sequence compensation in software, but you have to do it on the controller.
In this case on the controller that sits on the SCSI bus and controls the
floppy disk. Imagine a floppy disk sector track cut out an straightened to
look like this:

      o  x-0  x-1  x-2  x-3  x-4  x-5  x-6  x-7  x-8  x-9
      |                      |
      index hole             sector four (similar for others)

If the controller remembers the current position of the head after writing
sector 2, say, and you can turn the bus SCSI around and transfer sector 7
before that comes along, and the controller notices it and writes it out
your scheme works.

But if the controller waits for the index hole to come along again and then
starts counting until sector 7 comes along, you can do nothing but wait for
it, or abandon the floppy disk idea completely.

I hope this completes this particular thread, but if not, then please do
not type replies in yourself, use the telephone to dictate them for your
secretary to type as you dictate. This will give you an impression of the
type of clever things that can be done in software over the head of a
simple controller ....

                                      Thomas

--
*** This is the operative statement, all previous statements are inoperative.
*   email: cmaae47 @ ic.ac.uk (Thomas Sippel - Dau) (uk.ac.ic on Janet)
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