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    It takes 10 minutes to read 32Gb, but 3 hours to write the full pendrive with dd. All changes will not be more than 10Mbs in total. It would be great an utility for reading and comparing on the fly two block devices. Commented Feb 14, 2017 at 9:35
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    Oh, --devices is only for copying the device node file. Commented Jul 1, 2018 at 18:51
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    No, rsync may use timestamps but doesn't need to. You can tell rsync to ignore any timestamps which is often needed if files or whole directory trees are moved around without changing their timestamps. And according to arstechnica.com/civis/viewtopic.php?t=1173708 there exist patches to add the proper commandline switches to tell rsync to sync contents of block devices block by block if the blocks are different. Commented Nov 13, 2018 at 0:32
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    There is a case where rsync on the raw block devices is a lot faster and uses less resources than rsyncing the whole file system — if the file system contains many (millions) of hardlinks. This is common with e.g. BackupPC pools (at least with BackupPC 3.x). Also common with BackupPC pools is that for quite a while only additional blocks are used while most of the already used blocks stay the same. Depending on the network speed, it can be much quicker to just compare block hashsums instead of transfering TB of data again. Commented Nov 13, 2018 at 0:36
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    "That's usually slower than unconditionally doing the copy" - for local copies, yes sure. But for network copies it can still be faster to use rsync than cp/cat/dd, etc. Commented Oct 30, 2022 at 20:14