Re: writing a plugin for reiserfs compression

[Hans Reiser <reiser@namesys.com>]

Roy Sigurd Karlsbakk wrote:
> 
> > > I just thought there was a patch doing windows nt-like
> > > compress-em-all-realtime-and-get-doomed!
> >
> > I don't know what the actual heuristics for determining which files are
> > compression with the ext2 patch.  It is definitely NOT a compressed
> > block device.  Files are compressed in chunks (32kB?), so that it is
> > possible to seek and do read-modify-write (e.g. appending to a file)
> > without decompressing the entire file and/or recompressing it.  This also
> > protects against block corruption, since you would limit the amount of
> > data lost to the end of the chunk after the bad spot.
> 
> But still... Are the files are compressed as they are created/modified on
> the filesystem? My main point was to avoid the compression overhead and
> just decompress the file at access time if it's compressed. Compression
> should (IMO) be done nightly.
> 
> Perhaps a file should be decompressed when it's modified and either (a)
> set scheduled to next nightly compression or (b) stay uncompressed the
> next <n> days. I mean - as a file is being modified, the chance is large
> that the file will be accessed pretty soon...
> 
> > Yes, definitely disabling compression for a file is good.  The "accessed
> > in last 7 days flag" is questionable.  This could be determined via the
> > atime on Unix and doesn't need a separate flag.  Also, the difference
> > between "do not compress" and "can't compress" is very small.  If it is
> > found that the file is incompressible, you could just as easily set the
> > "do not compress" flag.
> 
> I agree on the 'accessed in last <n> days'. It'd be better to check atime.
> 
> I'd still like to separate 'do not compress' and 'can't compress', as to
> show why the falg has been set - the former is set by the admin and the
> latter by the system.
> 
> roy

I think you'll find it easiest to code it such that doing anything to the file
body, reading or writing, decompresses it.  Fancier stuff can wait for later
versions.

Hans