On Sun, 26 Jun 2005 15:54:25 PDT, Hans Reiser said:
> Valdis.Kletnieks@vt.edu wrote:
> 
> > (Hint - work out how long a kernel 'make' would take
> >if you were doing it inside a .tar.bz2).
> >  
> >
> After the first time, not very long, if you had enough ram....  the
> plugin would keep the data uncompressed until it flushed it to disk.

You're not allowed to use current existing stuff like the disk buffer cache
to weasel your way out on this one.  "if you had enough ram" has been true
for decades.  The trouble is that quite often you *don't* have enough ram....
 
> Performance might even improve since less would be written to disk.

I've worked with filesystems where performance improves due to compression
(AIX's JFS).  It's a lot harder to provide an improvement if you're writing
37 more bytes in between bytes 399457 and 399458.... (I suppose by aligning
byte 399458 so it actually is on the start of a 4K block you can do that, but
then you're losing the advantages of the compression.. ;)