Re: Transparent compression in the FS

[Andreas Dilger]

On Oct 15, 2003  13:19 -0400, Richard B. Johnson wrote:
> On Wed, 15 Oct 2003, Nikita Danilov wrote:
> > It could not if block-level compression is used. Which is the only
> > solution, given random-access to file bodies.
> 
> Then the degenerative case is no compression at all. There is no
> advantage to writing a block that is 1/N full of compressed data.
> You end up having to write the whole block anyway.

In the ext2 compression code, they compress maybe 8 source blocks into
(hopefully) some smaller number of compressed blocks.  Yes, there is still
a minimum block size, but you can save some reasonable fraction of the
total space (e.g. 8 blocks down to 4.5 blocks still gives you 5/8 = 37%
compression, although not 50%).  You get more efficient compression the
more source blocks you use, although your "damage area" grows in case of
error.

Cheers, Andreas
--
Andreas Dilger
http://sourceforge.net/projects/ext2resize/
http://www-mddsp.enel.ucalgary.ca/People/adilger/

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