How reliable is Unison? Did it ever ruin your data?

Question

I am interested in facts, when using unison ( http://www.cis.upenn.edu/~bcpierce/unison/ ) ruined your data? I want to find out about its reliability.

Answer 1

I stopped using Unison because:

• it can't handle special and international characters in a file name correctly. I think that these files were not copied over (but I am not sure about that).
• On a Mac, the (optional) GUI crashed often, so I had to restart the syncing process after each crash.

Answer 2

I have been using Unison on and off since something like 2004. In an answer to another question I gave it a nod over rsync as a tool for backing up/synchronizing your data between machines.

In all this time Unison never ruined my data in the sense of shredding file contents. It displayed, however, some sensitivity to edge conditions such as files in use, permissions, or cross-platform issues. You will need to be careful to research this if you encounter any errors when synchronizing your files with Unison. Save your logs.

A couple of weeks ago I decided to stop using Unison and went back to rsync. Main reasons:

• Unison is no longer actively developed, while rsync is
• Unison is slower than rsync in real-world usage, where I have hundreds of thousands of files totaling more than 150 GB in my home directory; a day's work's backup to an USB drive takes about 10 minutes with Unison but only 1-2 minutes with latest rsync.
• Unison's databases need rebuilding every couple of months due to aforementioned edge cases, such as sudden disconnection of receiving file system; when they are corrupted, your files will NOT be destroyed but they may remain unsynchronized and will give you weird errors. This database rebuild, especially with remote volumes, can take hours or even days.

Answer 3

I haven't been using it as long as ttarchala, but it does work nicely for smaller filesets and I have not lost any data.

While it is not under active development, it is being maintained to some degree. There have been updates/bugfixes committed to the source tree in the last few months, and you can get current binaries here (for example).

Also note that you can improve performance by setting fastcheck/pretendwin which detects file changes by size & date, rather than checksumming the entire file.

Answer 4

I've used Unison on my Macs for at least 8 years. I've never had Unison corrupt or lose a file. Early on, I had some issues with Unison not understanding resource forks, which led to failures to synchronize.

I started using Unison after I figured out that Finder on my Mac B&W G3 was silently corrupting copied files by randomly changing a byte or two every megabyte. (Caused by a hardware issue with Firewire on Rev 1 logic boards.) Since that problem, I've been really, really paranoid about comparing backup copies, and Unison does that well for me.

Answer 5

I used it for quite a while (to synch between desktop and laptop). As the others write, it is quite careful during synch, and I never lost any files. In case of problems it may require a (time-consuming) resynch, but everything sorts itself out in the end.

In regular operation, it is both fast and secure.