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portlandRyan
Asked: 2015-12-15 00:27:47 +0800 CST

Unison repeat=watch file limit?

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I'm running Ubuntu 14.04, when trying to sync a directory with unison using the option repeat=watch. The directory contains ~125K files but I'm abutting a limit on the number of files unison can handle. This is the error I receive:

Fatal error: Server: Filesystem watcher error: cannot add a watcher: system limit reached.
The watcher can be disabled by setting preference 'watch' to false.

I searched for more information by running unison with debug=all but these are the only lines I can find related to the error.

Any idea as to what system settings need to be changed to get around this limit?

unison

2 Answers

  1. I'm fairly certain that the problem isn't with Unison, but with inotify. When you use the option repeat=watch, Unison calls a helper program unison-fsmonitor which uses inotify to watch the filesystem for changes. It puts a "watch" on each file that you are syncing with Unison.

    By default there is an upper limit on the number of watches a single user can evoke. On my machine that upper limit is only 8192. You can increase the number of watches by editing the file

    /proc/sys/fs/inotify/max_user_watches

    I would be a bit concerned that since you want to watch about 125k files and the default maximum is only about 8k, you might see a dramatic hit in performance if you increase the maximum by that much.

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  2. You can add the line watch = false to your unison profile, and it should work, although without the "benefits" of the watcher.

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