COMPRESS (how much does it compress?)

COMPRESS (how much does it compress?)

Post by Alan McK » Sat, 13 Jun 1992 21:41:33



I was wondering if someone could tell me what the average compression
factor is for the compress command?  We are compressing some of the
filesystems in our nightly backups which makes it difficult to compute
how much data is actually being written to tape.  If I knew the average
compression factor then I could make better estimates.

The Sun man page says 50-60% for English text or source-code.  What
about for an entire filesystem?  What about for an oracle database?
What about other things like say binaries?

Does anyone know?

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COMPRESS (how much does it compress?)

Post by Dave Trinid » Wed, 17 Jun 1992 21:37:58



Quote:(Alan McKay) writes:
> I was wondering if someone could tell me what the average compression
> factor is for the compress command?  ....

The reason the man page is so vague is because the rate of compression is  
totally dependent on the data being compresses. Binaries do not compress all  
that well because they tend not to have patterers in the file that can be  
replaced by tokens. Usually the static data section at best. Text files
are full of spaces and words like "the" etc, which a deterministic algorithm  
can tokenize readily.

As far as your Oracle database ... 35% -> 40% seems to be an average I have  
encountered with compress.

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1. Compressed filesystem or Compressed loop?

I would like to create a compressed CVS repository, but I can not seem
to find a simple way to do this. . .  (Or really any way.)

I have seen people talk about the existence of a compressed loop.
What is this?  Does a compressed, read/write filesystem exist?

I found zisofs, but that looks like it would work read only, and
really only for a CD, i.e. the file tree needs to exist first then the
iso filesystem is created. . .  I found squashfs but that is also read
only.  If I create a loop-back device, i.e. a file that works like a
filesystem, I need to uncompress the file prior to mounting the loop.

Any ideas?

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