Can you clarify when data compression is used?

[Robert J. Hansen]

Kevin Hilton wrote:
> Although  not supported on all systems (and not included on ubuntu
> by default if you can believe it), does bzip2 offers the highest
> compression?

The question is meaningless.  It's predicated on the assumption that
there exists a ranking scheme by which bzip2 will always beat zip for
compression, or vice-versa.  The reality is that compression algorithms
have certain tasks they're good at and certain tasks they're awful at.

E.g., try compressing ciphertext sometime with either bzip2 or zip.  You
won't see any meaningful difference; both are equally awful at this.
Compressing PE/COFF versus ELF binaries will give different results.
Etc., etc., etc.

I can give only two bits of very broad advice, and one piece of specific
advice.  The two generals:

  * In most categories people care about, bzip2 offers better
    compression but is much slower.
  * Bandwidth is cheap.  It's not worth introducing interoperability
    problems just to get a slightly smaller file.

The one specific piece of advice:

  * Unless you can articulate a clear need why the defaults will not
    work for your purpose, stick with the defaults.