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Re: Debian's problems, Debian's future

On Wed, Apr 10, 2002 at 10:25:22AM +1000, Martijn van Oosterhout wrote:
> On Tue, Apr 09, 2002 at 05:02:34PM +0200, Michael Bramer wrote:
> > you propose to add 'some' diff files for all files on ftp-master.d.o? 
> > 
> > With rsync we need only one rsync-checksum file per normal file and
> > all apt's need only download the neededs parts.
> > 
> > You get the point?
> With the standard rsync algorithm, the rsync checksum files would actually
> be 8 times larger than the original file (you need to store the checksum
> for each possible block in the file).

I don't see that the checksum file is larger than the origanl file. If
the checksum file is larger, we will have more bytes to download... This
was not the goal.

> What you are suggesting is that the server store checksums for precalculated
> blocks on the server. This would be 4 bytes per 1k in the original file or
> so. The transaction proceeds as follows:
> 1. Client asks for checksum list off server
> 2. Client calculates checksums for local file
> 3. Client compares list of server with list of client
> 4. Client downloads changed regions.

Yes, this is the way..

> Note, this is not the rsync algorithm, but the one that is possibly
> patented.

maybe I don't understand the rsync algorithm...

IMHO the rsync algorithm is:
 1.) Computer beta splits file B in blocks.
 2.) calculate two checksums 
     a.) weak ``rolling'' 32-bit checksum
     b.) md5sum
 3.) Computer B send this to computer A.
 4.) Computer A search in file A for parts with the same checksums from
     file B
 5.) Computer A request unmatch blocks from computer B and 
     build the file B.

I get this from /usr/share/doc/rsync/tech_report.tex.gz


The _only_ difference is: precalculate the checksums on computer B

Or maybe store the calculated checksums in a /var/cache/rsync/ cache

sorry, I know that partentes don't have any logic, but this is the same
algorithm, only with some cache. Comments?

Michael Bramer  -  a Debian Linux Developer      http://www.debsupport.de
PGP: finger grisu@db.debian.org  -- Linux Sysadmin   -- Use Debian Linux
"Nicht geschehene Taten ziehen oft einen erstaunlichen Mangel an Folgen 
 nach sich."             --   S.J. Lec

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