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Can anyone clarify the differences between the --checksum and --ignore-times options of rsync?

My understanding is as follows:

If the file size and time match, it will do a checksum at both ends to see if the files are really identical.

'Transfer' every file, regardless of whether file time is same at both ends. Since it will still use the delta-transfer algorithm, if a file actually is identical, nothing gets transferred.

That's the technical difference, but as far as I can tell, they are semantically the same thing.

So, what I'm wondering is:

  • What is the practical difference between the two options?
  • In what cases would you use one rather than the other?
  • Is there any performance difference between them?
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4 Answers

Normally, rsync skips files when the files have identical sizes and times on the source and destination sides. This is a heuristic which is usually a good idea, as it prevents rsync from having to examine the contents of files that are very likely identical on the source and destination sides.

--ignore-times tells rsync to turn off the file-times-and-sizes heuristic, and thus unconditionally transfer ALL files from source to destination. rsync will then proceed to read every file on the source side, since it will need to either use its delta-transfer algorithm, or simply send every file in its entirety, depending on whether the --whole-file option was specified.

--checksum also modifies the file-times-and-sizes heuristic, but here it ignores times and examines only sizes. Files on the source and destination sides that differ in size are transferred, since they are obviously different. Files with the same size are checksummed (with MD5 in rsync version 3.0.0+, or with MD4 in earlier versions), and those found to have differing sums are also transferred.

In cases where the source and destination sides are mostly the same, --checksum will result in most files being checksummed on both sides. This could take long time, but the upshot is that the barest minimum of data will actually be transferred over the wire, especially if the delta-transfer algorithm is used. Of course, this is only a win if you have very slow networks, and/or very fast CPU.

--ignore-times, on the other hand, will send more data over the network, and it will cause all source files to be read, but at least it will not impose the additional burden of computing many cryptographically-strong hashsums on the source and destination CPUs. I would expect this option to perform better than --checksum when your networks are fast, and/or your CPU relatively slow.

I think I would only ever use --checksum or --ignore-times if I were transferring files to a destination where it was suspected that the contents of some files were corrupted, but whose modification times were not changed. I can't really think of any other good reason to use either option, although there are probably other use-cases.

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I've found --checksum useful along with --itemize-changes for verifying backups. Every now and again my backup scripts run a full compare this way after the current daily/weekly updates are complete. I get dropped an email marked urgent if --itemize-changes outputs anything unexpected, so I know there is a potential problem I should look into. –  David Spillett Sep 4 '12 at 9:58
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One detail: the checksum option checks a whole file on one end, then the whole file on the other end. If your files are somewhat big, this kind of kills parallelism.

Also, if you have huge files, you are most likely to run into a timeout with --checksum, as you don't with -I.

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From info rsync in regards to the --checksum option - "Since this whole-file checksumming of all files on both sides of the connection occurs in addition to the automatic checksum verifications that occur during a file's transfer, this option can be quite slow."

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That sentence doesn't seem to be in my man pages... so, does that imply that the checksum option will use checksums to identify whether the files are identical, and if they're not it will transfer, thus generating checksums again as part of the transfer? The --ignore-times option just skips the check and assumes they've changed? Therefore performance-wise --ignore-times is a better way of achieving the same thing? I'm still struggling to see why there are 2 different options (apart from the fact that --checksum is more transparent) –  Andy Madge Dec 9 '10 at 22:05
You should look at the latest documentation edit: gitweb.samba.org/… –  Aleksandr Levchuk Dec 9 '10 at 23:02
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The --ignore-times option will probably result in all files delta encoded and the delta-transfer algorithm (delta encoding) is at least as slow as the checksumming.

I don't know if rsync --ignore-times is smart enough to avoid the "automatic after-the-transfer verification" in the frequent case when the delta-transfer will result in nothing being transfered.

For --ignore-times:

  • In case rsync is not smart (or does not trust the delta encoding) then the checking (checksumming and encoding) will be done twice.
  • It could also be the case that delta encoding is much slower than the 128-bit MD4 checksumming.

Both --checksum and --ignore-times will be "quite slow" but --ignore-times is likely even slower (due to the 2 possibilities above).

Good question - please post if you find any performance differences in practice.

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I see what you mean. I'll run some tests and post back. –  Andy Madge Dec 9 '10 at 22:57
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