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From: Robert Dewar <robert_dewar@my-deja.com>
Subject: Re: Looking for keyed file package
Date: 1999/09/25
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In article <1999Sep24.130829.1@eisner>,
  Kilgallen@eisner.decus.org.nospam wrote:
ed key length of about 16 bits,
> > regardless of the actual physical key length.
>
> The VMS implementation of RMS does key compression and data
compression
> as well

Yes, but it uses a scheme basically similar to that used by
VSAM on IBM, the Realia COBOL scheme is significantly more
efficient, because it uses a single count to indicate both
the leading and trailing compression amounts.

> but I think that all started back when disk and memory space
> were much more dear.  I/O bandwidth one area of computer
> performance that does not seem to be advancing so much as the
> others, so these days typical advice might be to engage in
> extensive caching of the index data if performance is
> important.

The whole *point* of key compression is to allow this caching
to be more efficient. You want as much of the key index in main
memory as possible (and if you are really pushing things, in
cache as possible). Key compression is vitally important for
this, and in fact it is MORE important with modern architectures
than with older ones, precisely because of the increasing gap
between processor/memory speed and I/O speed.

Robert Dewar


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