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distribution tape compression

    Date: Thu, 15 Aug 1991 10:06 EDT
    From: s9274@srl1.LANL.GOV

	anyone know the relative tradeoffs of using compressed versus
	non-compressed files when writing dist tapes?

    Compression would also presumably be of use if you were trying to fit a
    large distribution on a single tape that would not fit uncompressed.
    Another nice feature of the compression substrate is that the
    distribution system handles decompression automatically.

This was a very strong motivation to Symbolics for our own use.  A
typical distribution compresses down to less than 50% of the size of the
uncompressed distribution.  It greatly reduces the number of tapes we
have to send out with a release.  So it can be very important if you
distribute a set of systems on tape that total more than 40MB or on
floppy (from a MacIvory) that total more than about 750KB.

    On the negative side, the compression and decompression is obviously
    comsuming extra time, but I have not noticed a significant slowdown in
    writing tapes. (I have not done any actual time comparisons.) As far as
    problems due to data loss, I have not experienced any to date.

We never did do a detailed study of the time penalty for that sizable
amount of compression.  I just did a quick test on a couple of small
distributions written to disk (300-500KB uncompressed).  The total
operation of writing the distribution from the Distribute Systems frame
(plan already built) took 14-20% longer for the compressed versions as
compared to writing the compressed ones.

This is in line with our general observation of the
distribution/restoration process for our own releases: the penalty for
using compression is so small that it should always be used for
distributions, without further thought, when it will save a tape or a