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Re: [rdiff-backup-users] rdiff-backup changes .gz files

From: Robert Nichols
Subject: Re: [rdiff-backup-users] rdiff-backup changes .gz files
Date: Sun, 17 Jul 2011 16:50:29 -0500
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On 07/17/2011 12:35 PM, Arno Wagner wrote:

I found a bit more time and found also some .pm files
with the same issue. Looking into the files though,
I have a suspicion, namely that for some reason these
files were changed by a Debian update, that then did
set the same original metadata after installing new
files. The suspicion comes from the differences being date
format changes only, no corruption at all.

So new question: Is there a way to make rdiff-backup
actually do a checksum comparison for exisitng files in
order to determine what to backup? I know, that would
be significantly slower, but as least on Debian system
partitions, a metadata comparison is obviously not
enough. As a substitute, can I force rdiff-backup
to backup specific files even if the metadata
is the same?

No way I know of to do that.  I find it surprising that .gz and .pm
files would be re-issued without changing the time stamps, but one
place where files do get changed without altering either the size or
modification time is during prelinking of ELF shared libraries and
binaries.  Now, the file size will increase the first time a file is
prelinked, and that change will be noticed by rdiff-backup.  But,
subsequent runs of prelink will just alter internal address fields,
so the size does not change.  In all cases the modification time is

Frankly, I view this as a mixed blessing.  If all of the altered
files got backed up every time prelink was run, my incremental
backups would balloon quite seriously, and it matters very little
which prelinked version gets restored.  The loader would detect the
out-of-date prelink information and do the same work it would have
to do with a non-prelinked binary, and everything will get back in
sync the next time prelink is run.  All that matters is that
rdiff-backup records a checksum that is consistent with the file it
_did_ back up, and that is always the case.

One thing that _does_ have a problem with these files is using rsync
to maintain two copies of an rdiff-backup archive, since rsync would
happily update metadata and increment files but fail to notice that
the mirror file itself had changed.  Avoiding use of the "-c" option
(which would be prohibitively expensive on a large archive) gets a
bit messy.

Bob Nichols     "NOSPAM" is really part of my email address.
                Do NOT delete it.

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