rsync/incementral

[steve harley]

they whom i call Ashley Aitken wrote:
>  From the article on macosxhints:

okay, i have read the lead on that page more carefully; it 
mentions hardlinks and it mentions RsyncX, but never quite puts 
them together to say that the script that RsyncX's Rotating 
Backup Assistant produces implements the complete 
hardlink/incremental scheme we've been discussing

so i ran the assistant, looked at the result and, by Jiminy, 
indeed it does it; your original assertion about RsyncX is 
correct, it's just that details of this feature are absolutely buried

the first major hint is in screen 4 of the assistant, which says 
"full backups that only consume disk space relative to the size 
of the changes", and then "each member 	of the backup rotation 
can be used for a 'restore from full backup'" (no explicit 
mention of the hardlinks)

the script is actually rather simple, and with RsyncX's help 
(which could be better, yes) it's probably easier to set up than, 
but also a bit more limited than, rsnapshot; here's the script 
for a rotation of 3 for example:

	#rotating backup script - v1.0
	rm -rf "/test2/folderset.2"
	mv -f "/test2/folderset.1" "/test2/folderset.2"
	mv -f "/test2/folderset.0" "/test2/folderset.1"
	time /usr/local/bin/rsync --rsync-path=/usr/local/bin/rsync -az 
--eahfs --showtogo --link-dest="/test2/folderset.1/" "/test1" 
"/test2/folderset.0/"

the assistant didn't actually manage to schedule the script, but 
that's a small hurdle

insofar as rsync is adequate i think RsyncX could help a lot of 
people do incremental backups; aside from the possibly irrelevant 
quirks of hardlinks, it's still worth noting the limitations of 
rsync; test of both the Tiger and RsyncX versions are in the 
comments here:

<http://www.n8gray.org/blog/2007/04/27/introducing-backup-bouncer>

the RsyncX version clearly preserves more (but not all) metadata; 
the discussion of Apple's request for Radar bugs makes me wonder 
if rsync will underlie Time Machine ...


> As I said, I thought rdiff-backup was a good option too.  I guess it 
> only provide a full version of the current backed-up directory 
> structure.  Rsync(X) provide a full version for each backup.
> 
> I hope that makes things clearer (as mud).

that's clear -- i just want to point out that in my experience 
this level of "directness" in incremental backups is unusual; 
other approaches are still "complete" though they require using a 
catalog of some sort to get to the files, or they require working 
through all the backup sets to find files that weren't changed 
recently; in this light rdiff-backup is a hybrid of sorts -- 
instant access to current mirror, but catalog used for past versions