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How does Duplicity determine if the local version of a file has been modified since the last backup? Does it only look for a different mtime/size etc., or does it actually check for modified content, by verifying some kind of content checksum (MD5 or SHA1)?

Is there some way to change that behavior? I want to make absolutely sure that all changes are included in a backup, even if by accident or malicious intent the modification date of a file is not properly updated; possibly without having to do a full backup every time.

lxgr
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2 Answers2

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Duplicity is based on librsync.

Rsync's defalt behaviour is to just look at the timestamp. I don't know wether Duplicity overrides this but you can add your own rsync options with the --rsync-options switch.

Add an option like 

--rsync-options="--checksum"

to your command and it should always use (the slower) checksumming to determin changed files.

Christopher Perrin
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  • Will that work with duplicity's architecture? There is no "intelligent" rsync at the remote end; does it store the checksums in an index tar archive for that purpose? Or will it have to read the backup files in over the network to determine the checksums of the remote files? – lxgr Jun 21 '12 at 17:29
  • rsync is on both ends AFAIK. It checks both files and compares the hashes. I don't think there is an index. – Christopher Perrin Jun 21 '12 at 17:33
  • That's impossible - Amazon S3 and even IMAP are storage options for duplicity, and there is no rsync available on either. – lxgr Jun 21 '12 at 17:56
  • Hmmm ... I think you should just try it with the options and see what happens. – Christopher Perrin Jun 21 '12 at 18:00
  • Unfortunately, it's hard to find a proper test case, but I'll try to find one. Thanks for the hint with rsync-options! – lxgr Jun 21 '12 at 19:46
  • this won't work according to the duplicity mailing list http://permalink.gmane.org/gmane.comp.sysutils.backup.duplicity.general/6057 – squiddle Nov 12 '13 at 09:14
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this wont work. see

http://lists.nongnu.org/archive/html/duplicity-talk/2012-11/msg00022.html
and
http://lists.nongnu.org/archive/html/duplicity-talk/2013-07/msg00010.html

..ede

ede-duply.net
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