docker checkpoint

内容説明

Checkpoint and Restore is an experimental feature that allows you to freeze a running container by specifying a checkpoint, which turns the container 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, 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.

Installing CRIU

If you use a Debian system, you can add the CRIU PPA and install with apt-get from the CRIU launchpad.

Alternatively, you can build CRIU from source.

You need at least version 2.0 of CRIU to run checkpoint and restore in Docker.

Use cases for checkpoint and 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 and 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 and restore

A new top level command docker checkpoint is introduced, with three subcommands:

• docker checkpoint create (creates a new checkpoint)
• docker checkpoint ls (lists existing checkpoints)
• docker checkpoint rm (deletes an existing checkpoint)

Additionally, a --checkpoint flag is added to the docker container start command.

The options for docker 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

Example of using checkpoint and 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 run docker logs in-between running/checkpoint/restoring, you should see that the counter increases while the process is running, stops while it's frozen, and resumes from the point it left off once you restore.

Known limitations

seccomp is only supported by CRIU in very up-to-date kernels.

External terminals (i.e. docker run -t ..) aren't supported. If you try to create a checkpoint for a container with an external terminal, it fails:

$ 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

サブコマンド