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29Mar/190

A journey from Unison to Syncthing and back to Unison

Posted by Avinash Meetoo

I generally work on my laptop, an old Dell Inspiron 15 still running Fedora Linux 29 quite well. At home, I also tend to work on an Apple iMac (still) running macOS El Capitan (which I intend to upgrade to High Sierra soon — unfortunately, Apple has decided that this iMac cannot run Mojave).

Since the very beginning, I needed to have a proper file synchronisation process for these two computers and cloud solutions such as Google Drive or Dropbox were not options as I had many many Gigabytes to synchronise and this would have been very expensive to be done online.

For 1-2 years, I used Unison, a file synchronisation utility written by one of my Computer Science heroes, Benjamin Pierce, author of the quite renowned book “Types and Programming Languages” which, I have to confess, I have not yet read because, well, I never had the chance to stumble upon it. Anyway, Unison is a tool which does two-way synchronisation between computers and every time there is an inconsistency (a new file, a deleted file, a change, etc.) ask the user to manually choose on the action to make. One important of Unison is that one has to run it manually i.e. the synchronisation only happens when initiated by the user. For example, I like to run Unison every afternoon when I get back home after one day of work.

One year ago, I stopped using Unison. The reason was that I wanted to have real time synchronisation between the two computers i.e. as soon as one changes, the other one is updated (if on of course.) I settled on Syncthing. I tried two different ways of using Syncthing (with a central repository or in peer-to-peer mode) and both worked as expected. But after one year, I have to say that I have two issues with Syncthing. First of all, changes are detected through the inotify mechanism provided by Linux and the macOS kernel. As soon as a file is changed, Syncthing knows about it and can propagate the change to the other computer. All good in principle. But this also means that all errors (a file deleted by mistake, a file overwritten by mistake, etc.) are also immediately propagated. So I used different kinds of versioning techniques to make sure that backups are kept in these situations. But, nevertheless, I slowly realised that real-time propagation might not be a good idea. The second issue is that Syncthing uses a lot of CPU.

Two days ago, I stopped the Syncthing service on all my computers and revived my Unison installation. And, believe it or not, Unison works great for my use case: I can synchronise my computers when I want (after work for instance) and I have the possibility (if there are incoherences) to think deep about which version to keep. For me, and because my files are very valuable to me, this is a much better process.

[At this point, let me point out that I use Unison forĀ synchronisation and I use regularĀ rsync for backup on other devices. It is important to understand the difference between these two processes. Synchronisation is not always needed but backups definitely. Especially when it is compliant with the 3-2-1 strategy.]

What about you? What do you use for synchronising multiple computers when cloud synchronisation is not possible? Do you use Unison? Syncthing? Something else? Why?