Re: [rdiff-backup-users] Backup not identifying changed file, even though hash is different

[Patrick Nagel]

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Hi,

On 2011-03-24 06:29, Scott Jilek wrote:
>>> Is there any way to force Rdiff to use a hash compare for difference
>>> triggering?  I looked over the manual, and nothing seems to be what I'm
>>> looking for.
>
> I've tried adding the "--compare-hash" switch.  The MAN page isn't specific
> about the function of --compare-hash, however it appears to be only for
> VERIFICATION and not for candidate IDENTIFICATION, as causes rdiff to simply
> show a report telling me this file is not in sync. When I use that switch no
> backups occur from what I can tell as no increments are created.
> 
> Is there some way to explicitly force Rdiff to create a backup of specific
> files?  What am I missing, because this seems like a huge gap in
> functionality if it's only using file timestamps.

I guess it would be nice to have an option to make rdiff-backup identify
changed files by checksum/hash. That being said, I think I wouldn't want to
use it for anything, because it would be extremely slow. It would need to
read all files from start to end, while hashing them, at each backup run.
For a large backup repository with, say, 500 GB worth of files, the backup
would take more than an hour, even if not a single file changed (assuming
100 MB/s read and hashing speed).

By the way, rsync also wouldn't consider that truecrypt container as
"changed", by default - you would need to add the '-c' option. And maybe
that's the best way to handle this: Exclude the truecrypt container(s) from
your rdiff-backup, and instead use rsync -c for that.
Alternatively you could 'touch' the truecrypt container just before the
backup, so that the modification timestamp gets changed, and rdiff-backup
will pick it up (but then it would create a diff every time, which would
take even longer than the 'rsync -c' method - but maybe you really want the
history of that file...).

Patrick.

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