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