RE: [rdiff-backup-users] Quick question - Can rdiff (or anything else) do this?

[Simon Hobson]

address@hidden wrote:

> Really?  My understanding of rsync & rdiff-backup was:  They will scan the
> source directory for changed files, and only copy the files that have
> changed.  My understanding was:  If a file has changed, the whole file
> will be sent (and not just the changed sections within the changed file.)
>
>
> Am I wrong about the 2nd point?  Can you show me some documentation to
> that fact?

Others have given links (and here's a good one: 
http://www.samba.org/rsync/how-rsync-works.html), but in a nutshell, 
Rsync will can the file at one end calculating checksums of blocks. 
At the other end, it will do a rolling checksum looking for matches - 
note that this is a rolling checksum so it will detect files that 
have had bits inserted or removed, thus altering the position of 
later data. It will then send the differences.

Thus, if you take a 1GB file and add a single byte at the beginning, 
rsync will only have to send the checksums (and hashes used for 
verification) plus the first block - it will detect that the rest of 
the file has simply moved along a byte.

Along with this, it can compress the data over the wire, use SSH to 
secure it, and even has the option to restrict bandwidth usage. Not 
only that, it runs multiple tasks in parallel - ie scans files 
looking for changes while still transferring earlier files. All this 
means that it is VERY effective over WAN links - like others, I've 
been using it for many years.

You mentioned OS X. If you use the version of Rsync provided by 
Apple, then it's patched to handle HFS extended attributes if you 
specify -E or --extended-attributes as an option.


--
Simon Hobson

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