Re: [rdiff-backup-users] rdiff-backup versus rsnapshot

[Richard Chapman]

Thanks Chris, Clement and Mike

I am posting this to both rdiff-backup group and the rsnapshot group so 
experts from each group can comment.

That seems to be a really good summary of the differences. I am now left 
with a tough choice - which really results from fundamental problems 
with heterogeneous networks - rather than any limitation of either tool.

It seems to me that rsnapshot has an "almost fatal" flaw if there is a 
need to store linux files on a windows file-system. If I understand 
correctly - it doesn't keep any separate metadata - so things like file 
ownership cannot be retained if (say) a backup repository for a linux 
system is kept on a windows system. There are probably also problems in 
the other direction.

Here is a wacky thought to work around this problem...
Might it be possible to use a "loop device" or something like that to 
create an ext3 file-system within a cifs file on a windows server? 
rsnapshot could then keep its backup repository in that ext3 
file-system. Would this solve the metadata problem when keeping a linux 
snapshot repository on a windows machine? Are there any obvious pitfalls 
in this approach? I guess windows couldn't see the backup files 
directly, but Linux could expose them as samba files if required.
I doubt there is a similar solution for storing windows files on a linux 
file-system?

rdiff-backup has a couple of "undesirable" features for my application. 
If I understand correctly - you cannot have backups kept at reducing 
frequency as they age. If you need (say) daily backups for short term 
reasons - you have to keep daily backups for as long as you want to keep 
backups. I guess the only way around this is to keep several different 
rdiff-backup repositories - with different frequencies and different 
final ages. This would greatly increase the storage requirement.

I am using maildir rather than mbox format for mail - so I guess there 
will be little advantage to either approach for mail system backup.

At restore time - especially after a major failure - rsnapshot has the 
advantage of a simple repository which can be restored with off-line 
tools etc. My wacky idea (above) probably reduces this simplicity.

Decisions... decisions...

Richard.






Chris Wilson wrote:
> Hi all,
>
> On Mon, 28 Apr 2008, Mike Marseglia wrote:
>
>   
> > A quick google turned up the following from
> > http://www.backupcentral.com/components/com_mambowiki/index.php/Rdiff-backup:
> >     
> ...
>   
> > Disadvantages
> >
> > Let’s be honest: rdiff-backup has some disadvantages too:
> >
> > Speed
> >
> >     rdiff-backup consumes more CPU than rsync and is therefore slower than
> > most rsync scripts. This difference is often not noticeable when the
> > bottleneck is the network or a disk drive but can be significant for
> > local backups.
> >     
>
> I would add the following:
>
> rdiff-backup uses a lot more network bandwidth than rsync. About 1GB per 
> 100GB covered per backup, in my estimate, in addition to the deltas. This 
> makes it too slow for us to use for daily offsite backups (uploading over 
> a 384kbit DSL, backing up about 500GB daily).
>
> rdiff-backup is a bit fragile. It's easy to corrupt the metadata, for 
> example if the store disk gets full, or multiple backups run to the same 
> destination at the same time, and usually impossible (i.e. nobody knows 
> how) to recover the history after that happens.
>
> rdiff-backup does not allow one to remove an intermediate increment (for 
> example if a large file accidentally got backed up that shouldn't be) or 
> to remove a subtree of the backup (at least not without risking metadata 
> corruption again).
>
>   
> >     With rsync scripts, all past backups appear as copies and are thus 
> > easy to verify, restore, and delete. With rdiff-backup, only the current 
> > backup appears as a true copy. (Earlier backups are stored as compressed 
> > deltas.)
> >     
>
> For me, this is a mixed blessing. Large numbers of small files take a lot 
> of space on the remote server (as with rsync too), and you can trash your 
> backup by accidentally modifying files in the remote repository. But it 
> has been useful in emergency recovery situations where I have had to boot 
> from a recovery CD that didn't have rdiff-backup on it.
>
> I do like rdiff-backup and I use it extensively, but these are things that 
> I wish for that would make it even better.
>
> Cheers, Chris.
>   
> ------------------------------------------------------------------------
>
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