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What does "use checksum to detect changes" do?

Asked by naught101

What exactly does the "use checksum to detect changes" option do? Does it make the backup *always* use a checksum? Or only for files that have identical modification times and sizes? Also, what is the behaviour when this option is off? And how much difference does it make to backup speed?

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Germar (germar) said : 		#1

Rsync normal behaviour is to compare mtime and size of src and dst to determine if a file has changed. If the file has changed it will create two checksums anyways. One rolling-checksum to find out which part of the file has changed and one normal.

If you use 'rsync --checksum' (which is done by activating 'use checksum...') rsync will calculate a checksum for every file and decide only on this if a file has changed. It will take ages...

I'd recommend to run snapshots with checksum periodically to make sure there is no corrupt file in your backup. Take a look at https://answers.launchpad.net/backintime/+faq/2445

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