93

I want to make a fresh new copy of a large number of files from one local drive to another.

I've read that rsync does a checksum comparison of files when sending them to a remote machine over a network.

  1. Will rsync make the comparison when copying the files between two local drives?

  2. If it does do a verification - is it a safe bet? Or is it better to do a byte by byte comparison?

7 Answers 7

102

rsync always uses checksums to verify that a file was transferred correctly. If the destination file already exists, rsync may skip updating the file if the modification time and size match the source file, but if rsync decides that data need to be transferred, checksums are always used on the data transferred between the sending and receiving rsync processes. This verifies that the data received are the same as the data sent with high probability, without the heavy overhead of a byte-level comparison over the network.

Once the file data are received, rsync writes the data to the file and trusts that if the kernel indicates a successful write, the data were written without corruption to disk. rsync does not reread the data and compare against the known checksum as an additional check.

As for the verification itself, for protocol 30 and beyond (first supported in 3.0.0), rsync uses MD5. For older protocols, the checksum used is MD4.

While long considered obsolete for secure cryptographic hashes, MD5 and MD4 remain adequate for checking file corruption.

Source: the man page and eyeballing the rsync source code to verify.

12
  • 8
    I hate to burst everyone’s bubble but rsync only does check sum verification if the -c flag is added!
    – user30825
    Jan 21, 2013 at 21:32
  • 34
    @clint No, the answer is correct. From the man page's explanation of the -c flag: "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." Jan 21, 2013 at 21:41
  • 15
    This answer does not make it clear if it actually verifies the file after a copy. If the checksum is computed as the file is being received, then it is not a post-copy checksum and you cannot be sure that the file is written correctly. You would then need to perform an additional comparison. Mar 24, 2015 at 21:26
  • 11
    Down-voting because I don't like the fact that this answer is detailed well written and technically correct and at the same time so much off topic that it misleads readers. The problem is that the answer goes into great detail on what happens during transfer while the questioner specifically states that he cares about local copies and not network transfers. I'm pretty sure Kyle Jones didn't want to mislead anyone but this answer (IMHO) does.
    – ndemou
    Jun 29, 2016 at 19:38
  • 7
    Kyle I don't believe you answer is wrong. I already noted it's "detailed well written and technically correct" but it requires the reader to be unnecessarily focused and careful. Why cover the lack of verification of the disk data which is being questioned halfway through your answer after 117 words which repeatedly describe an other irrelevant verification process? Anyway thanks for your time and interest in this discussion. I sincerely appreciate it.
    – ndemou
    Jul 1, 2016 at 8:43
52

rsync does not do the post-copy verification for local file copies. You can verify that it does not by using rsync to copy a large file to a slow (i.e. USB) drive, and then copying the same file with cp, i.e.:

time rsync bigfile /mnt/usb/bigfile

time cp bigfile /mnt/usb/bigfile

Both commands take about the same amount of time, therefore rsync cannot possibly be doing the checksum—since that would involve re-reading the destination file off the slow disk.

The man page is unfortunately misleading about this. I also verified this with strace—after the copy is complete, rsync issues no read() calls on the destination file, so it cannot be checksumming it. One more you can verify it is with something like iotop: you see rsync doing read and write simultaneously (copying from source to destination), then it exits. If it were verifying integrity, there would be a read-only phase.

5
  • 1
    "The man page is unfortunately misleading about this. I also verified this with strace" Did you strace the remote, running rsync process or the local one? There are two... one runs on the destination, even when you use ssh.
    – user129070
    May 6, 2013 at 19:20
  • 16
    There is no post-copy verification for any copies, local or remote. You run rsync -c again if you want to force it to check.
    – psusi
    May 6, 2013 at 23:50
  • 3
    The verification is done on the incoming stream as it goes. It's not necessary to read it back from the disk if the filesystem has confirmed it's been written.
    – OrangeDog
    Jul 11, 2018 at 15:51
  • 1
    rsync man page states "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." Feb 22, 2020 at 12:54
  • @openCivilisation, it looks correct, but so what? I myself got confused with that "receiving side" wording, just wrote an answer here for future reference. Feb 9 at 4:09
20

rsync makes a checksum comparison before copying (in some cases), to avoid copying what's already there. The point of the checksum comparison is not to verify that the copy was successful. That's the job of the underlying infrastructure: the filesystem drivers, the disk drivers, the network drivers, etc. Individual applications such as rsync don't need to bother with this madness. All rsync needs to do (and does!) is to check the return values of system calls to make sure there was no error.

7
  • 2
    This seems to contradict the accepted answer...
    – djule5
    Jan 13, 2016 at 6:45
  • 3
    @djule5 In what way? The accepted answer seems to mostly be about how rsync checks transferred files, but the question, and my answer, are about local copies. Jan 13, 2016 at 10:16
  • 3
    Ok, well in that context I agree it makes more sense. So "The point of the checksum comparison is not to verify that the copy was successful" is true only for local copies; and "checksums are always used on the data transferred between the sending and receiving rsync processes" is true only for transferred copies. I find the accepted answer misleading in regard to the question and believe your answer should be the accepted one (just my 2 cents).
    – djule5
    Jan 13, 2016 at 18:23
  • I still feel this answer is slightly misleading. For example, it says that the network drivers in particular verify if the copy was successful - but if you were saying that checksum comparison does not verify if the copy was successful for local only, network drivers would not come into play.
    – Ken
    Aug 7, 2017 at 19:56
  • 1
    @Ken I don't understand the point you're trying to make. I suspect you misread something. The network drivers come into play only if there's a network copy. Rsync itself does a checksum comparison before doing any copy, in order to decide whether to copy. Rsync doesn't do any checksum comparison after copying (because it would be pointless: it knows what it's just copied). Aug 7, 2017 at 20:04
14

Quick and dirty answers, directly to the questions.

Q: Will rsync make the comparison when copying the files between two local drives?

A: It will do comparison to figure out what to copy.

Q: If it does do a verification - is it a safe bet? Or is it better to do a byte by byte comparison?

A: as safe as the mathematics behind MD5 checksum of file. You can try to do simple experiment to learn and trust the tool.

Long answer: I guess, you wanted rsync to do file comparison (bit by bit or by checksum) after copying files. If you are one of the few that value data integrity, you might find the below useful:

rsync -avh [source] [destination] && rsync -avhc [source] [destination] 

The above code rsync files folder on first run and if complete without issue, will run rsync again immediately while performing same file name comparison by using hash of entire file.

7

Using rsync to verify the integrity of a duplicate

To guarantee that this test physically re-reads the files from the drive media, I suggest powering-down both drives and restarting them before running this test. This will clear their internal volatile caches.

If not also restarting Linux, you should at least drop the caches (*) with:

sudo sh -c 'echo 3 > /proc/sys/vm/drop_caches'

Then to re-read both trees and compare their checksums:

rsync --dry-run --checksum --itemize-changes --archive SRC DEST

Modern rsync checksum uses MD5, which is 128 bits. The likelihood of this failing to detect an error in an individual file is astronomically low (some discussion here), but not impossible.

5
0

Instead of using rsync you can use cp -rp to recursively copy a directory, followed by diff -r. GNU diff accepts two directory trees, which are both read in full when compared.

Rationale (thanks @they): both cp and diff are usually already installed and quite suitable for a onetime action like OP's "I want to make a fresh new copy of a large number of files from one local drive to another."

5
  • What is the reasoning behind using cp in place of rsync?
    – they
    Oct 23, 2021 at 11:34
  • 1
    It is already installed and quite suitable for a one-time action like OP's "I want to make a fresh new copy of a large number of files from one local drive to another." Oct 23, 2021 at 15:00
  • +1 for answering the requirement
    – roaima
    Oct 24, 2021 at 9:15
  • 1
    Note that by changing from rsync to cp, you are depending on the capabilities of the local cp utility to support the non-standard option -r, and you assume that it will do exactly what rsync would do (on some systems, the -r option would cause symbolic links to be followed, not copied as-is). You also remove the ability to copy hard links and sparse files (again, depending on the cp implementation), which would be done portably using rsync with its -H and --sparse options.
    – they
    Oct 24, 2021 at 9:55
  • Agreed that it is different. Point of this answer is: in many cases you don't need rsync but you can just get the job done with tools that are already installed. In case of hard links and sparse files, cp has options for those too, but those go beyond the requirement of the question which is to just copy a bunch of files from one local drive to another. BTW I have to disagree with the recursive flag being non-standard in the copy command. With diff that is different, which is why the answer states GNU diff. Oct 24, 2021 at 10:39
0

This answer is to clarify confusion I had myself and which is (IMO) difficult to understand from comments to accepted question. rsync man page states:

Note that rsync always verifies that each transferred file was correctly reconstructed on the receiving side by checking a whole-file check‐ sum that is generated as the file is transferred

It is probably correct, but it does not mean the actual written files data are checked after copy. First, with local drives both sides are one PC (so rsync might skip the check if side is the same); second, checksum "is generated as the file is transferred" (emphasis mine), not as being written to the media.

P.S. I've just investigated my finding that after copying with rsync I've got files with mismatched checksums (I copied on glitchy PC I knew that beforehand but from reading man page thought rsync would make sure files are copied properly) and found that QA.

Not the answer you're looking for? Browse other questions tagged or ask your own question.