Re: [rdiff-backup-users] Restoring files while rdiff-backup is running

[Simon Hobson]

Helge Milde wrote:

> Can you elaborate on this, please? I don't understand what's technically
> preventing rdiff-backup from letting me restore a backup from (for
> instance) a given date. As far as I understand, rdiff already has all
> the data it needs to do a restore, and since it should be a read-only
> operation, I don't understand how that would impact or corrupt the
> ongoing backup process.

OK, say you want to restore file A from some time ago.

rdiff-backup must take the current version of the file and apply all 
the rdiffs back to the version you need. So it starts with the 
current version of File A.

Two scenarios :

1) The backup process tries to update File A and finds it locked - 
backup fails.

2) The backup process doesn't find a locked file, but it managed to 
swap out the file for a new version just before the restore accesses 
it - but after it's accessed the metadata. The restore process now 
computes the old version of the file to restore whilst missing out 
the latest rdiff that it doesn't know about - you get a corrupt 
restored file.


Both of these need very precise timing to 'fail', but it's far easier 
to apply locking and be 100% sure it won't happen. The alternative 
would be file by file locking which would complicate things somewhat.

--
Simon Hobson

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