Docker

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This HOWTO page describes how to checkpoint and restore a Docker container.

Background

Starting with CRIU 1.3, it's possible to checkpoint and restore a process tree running inside a Docker container. However, it's important to note that Docker needs native support for checkpoint and restore in order to maintain its parent-child relationship and to correctly keep track of container states. In other words, while CRIU can C/R a process tree, the restored tree will not become a child of Docker and, from Docker's point of view, the container's state will remain "Exited" (even after successful restore).

Work is in progress to add native checkpoint and restore support to Docker. Once ready, specific commands (for example, "docker checkpoint" and "docker restore") will use CRIU to do the actual C/R operations while Docker continues to maintain its parent-child relationship and container states.

It's important to re-emphasize that by checkpointing and restoring a Docker container, we mean C/R of a process tree running inside a container, excluding the Docker daemon itself. As CRIU currently does not support nested PID namespaces, the C/R process tree cannot include the Docker daemon which runs in the global PID namespace.

Command Line Options

In addition to the usual CRIU command line options used when checkpointing and restoring a process tree, the following command line options are needed for Docker containers.

--root

This option has been used in the past only for restore operations that wanted to change the root of the mount namespace. It was not used for checkpoint operations.

However, because Docker by default uses the AUFS graph driver and the AUFS module in the kernel reveals branch pathnames in /proc/<pid>/map_files, --root is used to specify the root of the mount namespace. Once the kernel AUFS module is fixed, it won't be necessary to specify this option anymore.

--ext-mount-map

This option is used to specify the path of the external bind mounts. Docker sets up /etc/{hostname,hosts,resolv.conf} as targets with source files outside the container's mount namespace. Older versions of Docker also bind mount /.dockerinit.

For example, assuming the default Docker configuration, /etc/hostname in the container's mount namespace is bind mounted from the source at /var/lib/docker/containers/<container_id>/hostname.

--manage-cgroups

When a process tree exits after a checkpoint operation, the cgroups that Docker had created for the container are removed. This option is needed during restore to move the process tree into its cgroups, re-creating them if necessary.

--evasive-devices

Docker bind mounts /dev/null on /dev/stdin for detached containers (i.e., docker run -d ...). Since earlier versions of Docker used /dev/null in the global namespace, this option tells CRIU to treat the global /dev/null and the container /dev/null as the same device.

Restore Prework

As mentioned earlier, by default Docker uses AUFS to set up the container's filesystem. When Docker notices that the process has exited (due to criu dump), it dismantles the filesystem. We need to set up the filesystem again before attempting to restore.

An Example

Below is an example to show C/R operations for a shell script that continuously appends a number to a file. You can use tail -f to see the process in action.

As you will see below, after restore, the process's parent is PID 1 (init), not Docker. Also, although the process has been successfully restored, Docker still thinks that the container has exited.

To set up the container's AUFS filesystem before restore, its branch information should be saved before checkpointing the container. For convenience, however, AUFS branch information is saved in the dump.log file. So we can examine dump.log to set up the filesystem again.

For brevity, the 64-character long container ID is replaced by the string <container_id> in the following lines.

$ docker run -d busybox:latest /bin/sh -c 'i=0; while true; do echo $i >> /foo; i=$(expr $i + 1); sleep 3; done'
<container_id>
$ 
$ docker ps
CONTAINER ID  IMAGE           COMMAND           CREATED        STATUS
168aefb8881b  busybox:latest  "/bin/sh -c 'i=0; 6 seconds ago  Up 4 seconds
$ 
$ sudo criu dump -o dump.log -v4 -t 17810 \
	-D /tmp/img/<container_id> \
	--root /var/lib/docker/aufs/mnt/<container_id> \
	--ext-mount-map /etc/resolv.conf:/etc/resolv.conf \
	--ext-mount-map /etc/hosts:/etc/hosts \
	--ext-mount-map /etc/hostname:/etc/hostname \
	--ext-mount-map /.dockerinit:/.dockerinit \
	--manage-cgroups \
	--evasive-devices
$
$ sudo grep successful /tmp/img/<container_id>/dump.log
(00.020103) Dumping finished successfully
$
$ docker ps -a
CONTAINER ID  IMAGE           COMMAND           CREATED        STATUS
168aefb8881b  busybox:latest  "/bin/sh -c 'i=0; 6 minutes ago  Exited (-1) 4 minutes ago
$
$ sudo mount -t aufs -o br=\
/var/lib/docker/aufs/diff/<container_id>:\
/var/lib/docker/aufs/diff/<container_id>-init:\
/var/lib/docker/aufs/diff/a9eb172552348a9a49180694790b33a1097f546456d041b6e82e4d7716ddb721:\
/var/lib/docker/aufs/diff/120e218dd395ec314e7b6249f39d2853911b3d6def6ea164ae05722649f34b16:\
/var/lib/docker/aufs/diff/42eed7f1bf2ac3f1610c5e616d2ab1ee9c7290234240388d6297bc0f32c34229:\
/var/lib/docker/aufs/diff/511136ea3c5a64f264b78b5433614aec563103b4d4702f3ba7d4d2698e22c158:\
none /var/lib/docker/aufs/mnt/<container_id>
$
$ sudo criu restore -o restore.log -v4 -d
	-D /tmp/img/<container_id> \
	--root /var/lib/docker/aufs/mnt/<container_id> \
	--ext-mount-map /etc/resolv.conf:/var/lib/docker/containers/<container_id>/resolv.conf \
	--ext-mount-map /etc/hosts:/var/lib/docker/containers/<container_id>/hosts \
	--ext-mount-map /etc/hostname:/var/lib/docker/containers/<container_id>/hostname \
	--ext-mount-map /.dockerinit:/var/lib/docker/init/dockerinit-1.0.0 \
	--manage-cgroups \
	--evasive-devices
$
$ sudo grep successful /tmp/img/<container_id>/restore.log
(00.424428) Restore finished successfully. Resuming tasks.
$
$ ps -ef | grep /bin/sh
root     18580     1  0 12:38 ?        00:00:00 /bin/sh -c i=0; while true; do echo $i >> /foo; i=$(expr $i + 1); sleep 3; done
$
$ docker ps -a
CONTAINER ID  IMAGE           COMMAND           CREATED        STATUS
168aefb8881b  busybox:latest  "/bin/sh -c 'i=0; 7 minutes ago  Exited (-1) 5 minutes ago
$