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From: mcculley@alien.enet.dec.com
Newsgroups: comp.realtime
Subject: Re: real-time multicomputer systems
Message-ID: <11986@shlump.nac.dec.com>
Date: 25 May 90 18:33:26 GMT
Sender: newsdaemon@shlump.nac.dec.com
Organization: Digital Equipment Corporation
Lines: 39


In article <9005241526.AA12356@fsucs.cs.fsu.edu>, groh@fsucs.cs.fsu.edu (Jim Groh) writes...
>In article <4228@hcx1.SSD.CSD.HARRIS.COM>, steve@SSD.CSD.HARRIS.COM (Steve Brosky) writes:
>>
>>* contiguous disk files --
>>	allows faster disc I/O because seeks are eliminated
> 
>          Isn't that also dependent on file size and disk configuration?
> 

Not only that, it makes a lot of assumptions about the nature of the 
disk i/o (IMHO questionable ones at that).  Basically, seeks can be
eliminated only for the file blocks contained in the track(s)/cylinder(s)
presently under the heads (and maybe the next in sequence).  So the elimination
of seeks will be true only for one limited portion of one (possibly large?)
file.  Access to other files, and other parts of that file, will require seeks.
The overhead involved in accomplishing those seeks may or may not be increased
due to the simplistic contiguous file structure (in my experience Murphy will
insist that it is).

There is a related assumption that there will be only one single access stream
to a disk at any time, or else seeks will still be required.  So that
multiprocessor configuration either needs a disk per processor, or some of them
will have to wait for that single access stream to become available in order to
hit the disk without causing those dreaded seeks.

And I won't ask about extending files...

Contiguous files are a very simple structure, as such they minimize overhead at
the expense of capabilities.  Might be the best thing for a system requiring
only one disk access stream, on the other hand if you have multiple processors
wouldn't multiple disk access streams be nice too?  Like any tradeoff, you pays
your money, you takes your choice.


- Bruce McCulley
  RSX-11 Software Development
  Digital Equipment Corp.
  Nashua, NH USA