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I want to move some files using rsync, but it's vital that the files are intact at the destination before removing the source files. The thing is, I can't find any information telling me how rsync decides that a file has been transferred correctly: I'll need it to be properly checksummed. Anyone know how rsync does it?

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  • I am pretty confident that if it can't pass any attributes for instance, it won't accept the file as being copied. succesfully But why don't you give it a try by killing the session mid-copy? After all the algorithm might be the same but the implementations differ.
    – user
    Dec 20, 2011 at 14:24
  • In answer to "how safe is it": how good is your disaster recovery plan?
    – Andrew
    Dec 21, 2011 at 4:22
  • Rock solid on one side of the sync, non-existant at the other. And unfortunately I don't have control over that side :-( Hence the reason I'd like to make any failures as unlikely as possible. Dec 21, 2011 at 11:29

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You need to pass the --checksum to ensure files that have the same size & times on source and destination are not skipped. Then, as per Wikipedia:

The recipient splits its copy of the file into fixed-size non-overlapping chunks and computes two checksums for each chunk: the MD4 hash, and a weaker 'rolling checksum'. (Version 30 of the protocol, released with rsync version 3.0.0, now uses MD5 hashes rather than MD4.[14]) It sends these checksums to the sender.

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The section about --checksum in the manpage says rsync always verifies the ckecksum of the received file to consider it complete (even when --checksum is not used):

Note that rsync always verifies that each transferred file was correctly reconstructed on the receiving side by checking a whole-file checksum that is generated as the file is transferred, but that automatic after-the-transfer verification has nothing to do with this option's before-the-transfer "Does this file need to be updated?" check.

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