Syncing many files with unison

[mre]

File sync didn't work properly on bigger projects (>100.000 files).
Sometimes files were missing inside the guest or on the host.
After some investigation I noticed that the unison container was crashing from time to time.

Looking at the container logs, I found the following error message:

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

I am using docker-machine + boot2docker.

The following commands fixed the issue:

1. Log into boot2docker

docker-machine ssh

2. Increase the number of filesystem watches that unison can start

Add the following line to the end of /var/lib/boot2docker/profile:

sudo sysctl fs.inotify.max_user_watches=524288

3. Restart docker machine

docker-machine restart

After that, syncing bigger projects started to work for me.
This is not a support ticket.
It's a limitation of boot2docker, but I still wanted to document the behaviour somewhere, in case anybody else is running into the same problem.

[michaelsauter]

Nice find. Thanks a lot for the report. I might add the issue to a "Troubleshooting" section, and a general indication how to solve it. I never tried Crane with docker-machine, I'm using it with Docker for Mac and a project with ~50k files.