compression, built-in or ssh ?

jw schultz jw at
Fri Oct 17 18:00:54 EST 2003

On Fri, Oct 17, 2003 at 09:34:17AM +0200, Lapo Luchini wrote:
> jw schultz wrote:
> >>What is the general recommendation for compression when using ssh?
> >>   
> >>
> >Use rsync's compression.
> > 
> >
> >>If so, is there a clear prefference which is more efficient, rsync or ssh?
> >>   
> >>
> >Yes.
> > 
> >
> Why, if they both use zlib?
> Moreover compressing "at a higher level" always seems a good diea to me 
> (e.g. if you compress the whole SSH session, you're sure *anything* is 
> compressed, if yuo let each application encrypt you have to nkow each 
> application, and maybe it does compress only data and not commands, just 
> to do an example).
> In the case of rsync I always thought it was the same, and I never use 
> -z on rsync as I have compression on by default on SSH connections.
> Any actual reason not to do that?

For the file data sent rsync seeds the compressor so that it
achieves a higher level of compression than can be achieved
my only compressing the blocks transmitted.

Whether you use the -z option or not rsync is
micro-optimised in transmitting the file list so that the
meta-data transmission is effectively compressed.  About the
only thing compressible that is not compressed are error

Were it not for the micro-optimisation i would say that
which level to compress for maximum effect would depend on
the amount of non-matched data sent.

One way that doing the compression in ssh does have an
advantage is that with ssh protocol version 1 you can
control the compression level.

	J.W. Schultz            Pegasystems Technologies
	email address:		jw at

		Remember Cernan and Schmitt

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