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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.

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this wont work. see


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Welcome to Server Fault! Generally we like answers on the site to be able to stand on their own - Links are great, but if that link ever breaks the answer should have enough information to still be helpful. Please consider editing your answer to include more detail. See the FAQ for more info. – slm Jul 22 '13 at 14:41

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


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

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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

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