Why does docker-machine clear data on restart?

I'm using Docker Toolbox on OSX.

I've created a data volume container for storing persistent data: https://docs.docker.com/userguide/dockervolumes/#creating-and-mounting-a-data-volume-container.

I checked that this data is indeed stored on the boot2docker VM (created by docker-machine) and not on the container, so that it will persist. However "docker-machine restart " clears out this custom data on the vm.

I can't find documentation on what is happening. I found one forum post mentioning that data in /var/lib/docker will be preserved, but I could not find any official docs stating that and it also seems strange considering the persistent storage guide above doesn't use this path or even mention that your data will be deleted.

Is this expected and if so is there any official documentation on the correct path to store persistent data?

Edit: Adding example of failing senario

$ docker-machine ssh alt
docker@alt:~$ docker run -v /data:/var/lib/mysql --name mydata busybox sh -c "echo 'hello' > /var/lib/mysql/hello"
docker@alt:~$ docker run --rm --volumes-from mydata busybox sh -c "cat /var/lib/mysql/hello"
hello
docker@alt:~$ exit
$ docker-machine restart alt
Starting VM...
$ docker-machine ssh alt
docker@alt:~$ docker run --rm --volumes-from mydata busybox sh -c "cat /var/lib/mysql/hello"
cat: can't open '/var/lib/mysql/hello': No such file or directory

Solution 1:

This definitely should work:

$ docker-machine ssh default
docker@default:~$ docker run -v /data --name mydata busybox true
docker@default:~$ docker run --volumes-from mydata busybox sh -c "echo hello >/data/hello"
docker@default:~$ docker run --volumes-from mydata busybox cat /data/hello
hello
docker@default:~$ exit
$ docker-machine restart default
Starting VM...
$ docker-machine ssh default
docker@default:~$ docker run --volumes-from mydata busybox cat /data/hello
hello

Can you elaborate more on the steps to reproduce your problem?

boot2docker has a read-only filesystem (will get wiped on reboot) with the exception of:

1. Containers and their data (volumes) -- this is what you read about /var/lib/docker
2. Docker images
3. Docker configuration (e.g. /var/lib/boot2docker/profile where the daemon flags can be tweaked)

Solution 2:

I don't use boot2docker, but if /data gets wiped on reboot, that's where your volume is being stored (docker run -v /data:/var/lib/mysql), so it will be lost.

What you're doing is also combining two different patterns for handling volume persistence. In order to get persistence, containers can mount volumes from a specified location on the host system (which is presumed to be persistent), or they can be associated with a data container, and mounted with --volumes-from. It sounds like the host filesystem approach is not appropriate for boot2docker, and you should use the data volume pattern (only).

Presumably you should be creating your data container with -v /var/lib/mysql, rather than -v /data:/var/lib/mysql.