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[Duplicity-talk] Re: [Duplicity-team] [Question #116587]: Can the "verif

From: Peter Schuller
Subject: [Duplicity-talk] Re: [Duplicity-team] [Question #116587]: Can the "verify" option be used to ensure the entire backup could restore without error?
Date: Tue, 6 Jul 2010 18:19:44 +0200

[kenneth responded but the question didn't end up on cc; quoting him entirely]

> Going to jump in here rather than later in the thread where its messy.
> Duplicity does verify the contents of the archives *as they were*, it
> does not do a comparison with the contents on the filesystem.  Verify is
> done by comparing the archive contents with the stored signatures, i.e.
> the original file with its hash value.
> The assumption is that the filesystem will probably change shortly after
> backup.  What you look for in a verify is a check to see if the backup
> is stored properly and can be restored.  If you want a comparison
> function, you'll need to restore and compare the original with the
> restored files, or provide a direct comparison function for us to
> integrate into duplicity.
> If you want to test verify, backup to a local file system, hexedit one
> of the archives and try to verify.  It will fail to verify.  You can
> modify the original files at will, and verify will succeed, as it is
> designed to do.

Ok. The fact that it even reported discrepancies at all (with meta
data) always gave me the impression it was intended to actually
compare. And this is not at all useless since a good backup procedure
will typically involve, where possible,  backing up data from a
snapshot of a file system rather than a live file system, in which
case such behavior would indeed be useful.

But ok - so verify does, we still believe, verify that the backup has
not been corrupted along the way, but makes no claims to compare
against actual file system contents (even when that contents may be
matching exactly).

In short then, 'verify' verifies that whatever was backed up still
seems to be modified, but does not really try to verify the
correctness of that backup?

I wonder though it not more people are under my mistaken impression
since the meta data discrepancies are clearly reported. If the intent
of verify is just to verify internal integrity, why is a file system
even involved in the process (i.e., why even compare a file system
hierarchy at all)?

Oh well.

/ Peter Schuller

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