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How does rsync work?

319 pointsby securealmost 3 years ago

8 comments

throw0101aalmost 3 years ago
This is also available as a video, &quot;Why I wrote my own rsync&quot;:<p>* <a href="https:&#x2F;&#x2F;media.ccc.de&#x2F;v&#x2F;gpn20-41-why-i-wrote-my-own-rsync" rel="nofollow">https:&#x2F;&#x2F;media.ccc.de&#x2F;v&#x2F;gpn20-41-why-i-wrote-my-own-rsync</a><p>* <a href="https:&#x2F;&#x2F;www.youtube.com&#x2F;watch?v=wpwObdgemoE" rel="nofollow">https:&#x2F;&#x2F;www.youtube.com&#x2F;watch?v=wpwObdgemoE</a>
boomskatsalmost 3 years ago
This was a great write up. I&#x27;ve already sent it to a few people.<p>On the question of what happens if a file&#x27;s contents change after the initial checksum, the man page for rsync[0] has an interesting explanation of the *--checksum* option:<p>&gt; This changes the way rsync checks if the files have been changed and are in need of a transfer. Without this option, rsync uses a &quot;quick check&quot; that (by default) checks if each file&#x27;s size and time of last modification match between the sender and receiver. This option changes this to compare a 128-bit checksum for each file that has a matching size. Generating the checksums means that both sides will expend a lot of disk I&#x2F;O reading all the data in the files in the transfer (and this is prior to any reading that will be done to transfer changed files), so this can slow things down significantly.<p>&gt; The sending side generates its checksums while it is doing the file-system scan that builds the list of the available files. The receiver generates its checksums when it is scanning for changed files, and will checksum any file that has the same size as the corresponding sender&#x27;s file: files with either a changed size or a changed checksum are selected for transfer.<p>&gt; 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&#x27;s before-the-transfer &quot;Does this file need to be updated?&quot; check. For protocol 30 and beyond (first supported in 3.0.0), the checksum used is MD5. For older protocols, the checksum used is MD4.<p>[0]: <a href="https:&#x2F;&#x2F;linux.die.net&#x2F;man&#x2F;1&#x2F;rsync" rel="nofollow">https:&#x2F;&#x2F;linux.die.net&#x2F;man&#x2F;1&#x2F;rsync</a>
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lazypenguinalmost 3 years ago
Nice write up. rsync is great as an application but I found it more cumbersome to use when wanting to integrate it into my own application. There&#x27;s librsync but the documentation is threadbare and it requires an rsync server to run. I found bita&#x2F;bitar (<a href="https:&#x2F;&#x2F;github.com&#x2F;oll3&#x2F;bita" rel="nofollow">https:&#x2F;&#x2F;github.com&#x2F;oll3&#x2F;bita</a>) which is inspired by rsync &amp; family. It works more like zsync which leverages HTTP Range requests so it doesn&#x27;t require anything running on the server to get chunks. Works like a treat using s3&#x2F;b2 storage to serve files and get incremental differential updates on the client side!
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lloekialmost 3 years ago
When trying to understand rsync and the rolling checksum I stumbled upon a small python implementation in some self-hosted corner of the web[0], which I have archived on GH[1] (not the author, but things can vanish quickly, as proved by the bzr repo which went <i>poof</i>[2]).<p>[0]: <a href="https:&#x2F;&#x2F;blog.liw.fi&#x2F;posts&#x2F;rsync-in-python&#x2F;" rel="nofollow">https:&#x2F;&#x2F;blog.liw.fi&#x2F;posts&#x2F;rsync-in-python&#x2F;</a><p>[1]: <a href="https:&#x2F;&#x2F;github.com&#x2F;lloeki&#x2F;rsync&#x2F;blob&#x2F;master&#x2F;rsync.py" rel="nofollow">https:&#x2F;&#x2F;github.com&#x2F;lloeki&#x2F;rsync&#x2F;blob&#x2F;master&#x2F;rsync.py</a><p>[2]: <a href="https:&#x2F;&#x2F;code.liw.fi&#x2F;obsync&#x2F;bzr&#x2F;trunk&#x2F;" rel="nofollow">https:&#x2F;&#x2F;code.liw.fi&#x2F;obsync&#x2F;bzr&#x2F;trunk&#x2F;</a>
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throw0101aalmost 3 years ago
See also the 1996 original paper by Tridgell (also of Samba fame) and Mackerras:<p>* <a href="https:&#x2F;&#x2F;rsync.samba.org&#x2F;tech_report&#x2F;" rel="nofollow">https:&#x2F;&#x2F;rsync.samba.org&#x2F;tech_report&#x2F;</a><p>* <a href="https:&#x2F;&#x2F;www.andrew.cmu.edu&#x2F;course&#x2F;15-749&#x2F;READINGS&#x2F;required&#x2F;cas&#x2F;tridgell96.pdf" rel="nofollow">https:&#x2F;&#x2F;www.andrew.cmu.edu&#x2F;course&#x2F;15-749&#x2F;READINGS&#x2F;required&#x2F;c...</a>
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srvmshralmost 3 years ago
I encountered a strange situation 2 days ago. I rsync my pdf files periodically between my harddrives. rsync showed no differences between two folder trees, but if I did `diff -r` between the two, 3 pdfs came out different.<p>I checked the three individually but they showed no corruption or changes either side. How can this happen?<p>Edit: the hard drive copy is previously rsynced from this copy &amp; both copies are mirrored with google cloud bucket.<p>The 3 files which showed different have the same MD5 checksum
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CamperBob2almost 3 years ago
I don&#x27;t see why any of this is needed. Just install Dropbox, and...
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bigChrisalmost 3 years ago
Rsync worst issue is someone port scanning and brute force their way into your system. Turn off your port.
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