# Docker Checkpoint & Restore Checkpoint & Restore is a new feature that allows you to freeze a running container by checkpointing it, which turns its state into a collection of files on disk. Later, the container can be restored from the point it was frozen. This is accomplished using a tool called [CRIU](http://criu.org), which is an external dependency of this feature. A good overview of the history of checkpoint and restore in Docker is available in this [Kubernetes blog post](http://blog.kubernetes.io/2015/07/how-did-quake-demo-from-dockercon-work.html). ## Installing CRIU If you use a Debian system, you can add the CRIU PPA and install with apt-get [from the criu launchpad](https://launchpad.net/~criu/+archive/ubuntu/ppa). Alternatively, you can [build CRIU from source](http://criu.org/Installation). You need at least version 2.0 of CRIU to run checkpoint/restore in Docker. ## Use cases for checkpoint & restore This feature is currently focused on single-host use cases for checkpoint and restore. Here are a few: - Restarting the host machine without stopping/starting containers - Speeding up the start time of slow start applications - "Rewinding" processes to an earlier point in time - "Forensic debugging" of running processes Another primary use case of checkpoint & restore outside of Docker is the live migration of a server from one machine to another. This is possible with the current implementation, but not currently a priority (and so the workflow is not optimized for the task). ## Using checkpoint & restore A new top level command `docker checkpoint` is introduced, with three subcommands: - `create` (creates a new checkpoint) - `ls` (lists existing checkpoints) - `rm` (deletes an existing checkpoint) Additionally, a `--checkpoint` flag is added to the container start command. The options for checkpoint create: Usage: docker checkpoint create [OPTIONS] CONTAINER CHECKPOINT Create a checkpoint from a running container --leave-running=false Leave the container running after checkpoint --checkpoint-dir Use a custom checkpoint storage directory And to restore a container: Usage: docker start --checkpoint CHECKPOINT_ID [OTHER OPTIONS] CONTAINER A simple example of using checkpoint & restore on a container: $ docker run --security-opt=seccomp:unconfined --name cr -d busybox /bin/sh -c 'i=0; while true; do echo $i; i=$(expr $i + 1); sleep 1; done' > abc0123 $ docker checkpoint create cr checkpoint1 # <later> $ docker start --checkpoint checkpoint1 cr > abc0123 This process just logs an incrementing counter to stdout. If you `docker logs` in between running/checkpoint/restoring you should see that the counter increases while the process is running, stops while it's checkpointed, and resumes from the point it left off once you restore. ## Current limitation seccomp is only supported by CRIU in very up to date kernels. External terminal (i.e. `docker run -t ..`) is not supported at the moment. If you try to create a checkpoint for a container with an external terminal, it would fail: $ docker checkpoint create cr checkpoint1 Error response from daemon: Cannot checkpoint container c1: rpc error: code = 2 desc = exit status 1: "criu failed: type NOTIFY errno 0\nlog file: /var/lib/docker/containers/eb62ebdbf237ce1a8736d2ae3c7d88601fc0a50235b0ba767b559a1f3c5a600b/checkpoints/checkpoint1/criu.work/dump.log\n" $ cat /var/lib/docker/containers/eb62ebdbf237ce1a8736d2ae3c7d88601fc0a50235b0ba767b559a1f3c5a600b/checkpoints/checkpoint1/criu.work/dump.log Error (mount.c:740): mnt: 126:./dev/console doesn't have a proper root mount