Docker
Debugging scratch containers
Containers made from scratch images have no /bin/sh or anything except for the binary for the container to run.
To debug it, spin up a separate container using, say, alpine image and have it spawn in the same namespace as the container you’re trying to debug:
$ docker run -it --rm --pid=container:60c1c49379cf --net=container:60c1c49379cf alpine sh
Unable to find image 'alpine:latest' locally
latest: Pulling from library/alpine
2aecc7e1714b: Pull complete
Digest: sha256:0b94d1d1b5eb130dd0253374552445b39470653fb1a1ec2d81490948876e462c
Status: Downloaded newer image for alpine:latest
/ # ps
PID USER TIME COMMAND
1 root 0:04 /traefik --configfile=/config/traefik.toml
10 root 0:00 sh
14 root 0:00 ps
/ #
To have the same root file system, add --volumes-from 60c1c49379cf.
Here for more details
Monitor unix domain socket
sudo mv /path/to/sock /path/to/sock.original
sudo socat -t100 -x -v UNIX-LISTEN:/path/to/sock,mode=777,reuseaddr,fork UNIX-CONNECT:/path/to/sock.original
Delete all stopped containers
docker rm $(docker ps -aq)
Delete all untagged images
docker rmi $(docker images | grep "^<none>" | awk '{print $3}')
Remove dangling images
docker rmi -f $(docker images -q -a -f dangling=true)
Remove dangling volumes
docker volume ls -qf dangling=true | xargs -r docker volume rm
Docker development notes
To run the dev container:
docker run --privileged --rm -ti -v `pwd`:/go/src/github.com/docker/docker dry-run-test /bin/bash
To run a named test:
TESTFLAGS='-test.run ^TestValidateIPAddress$' make test-unit
if running inside the container, replace make with hack/make.sh
Other
Use exec in wrapper scripts
Many images use wrapper scripts to do some setup before starting a process for the software being run. It is important that if your image uses such a script, that script should use exec so that the script’s process is replaced by your software. If you do not use exec, then signals sent by docker will go to your wrapper script instead of your software’s process. This is not what you want - as illustrated by the following example:
Say that you have a wrapper script that starts a process for a server of some kind. You start your container (using docker run -i), which runs the wrapper script, which in turn starts your process. Now say that you want to kill your container with CTRL+C. If your wrapper script used exec to start the server process, docker will send SIGINT to the server process, and everything will work as you expect. If you didn’t use exec in your wrapper script, docker will send SIGINT to the process for the wrapper script - and your process will keep running like nothing happened.
From http://www.projectatomic.io/docs/docker-image-author-guidance/
Running out of space
This seems to happen more often on Docker 0.12 using overlay storage engine.
The operating system reports that there’s no space left on the device, even though df shows that there are plenty of space left.
It turns out that No space left on device may also be caused by not having enough metadata blocks. Use df -i to check the available inodes:
root@ip-172-31-51-118:~# df -i
Filesystem Inodes IUsed IFree IUse% Mounted on
udev 124890 393 124497 1% /dev
tmpfs 126805 523 126282 1% /run
/dev/xvda1 524288 524288 0 100% /
tmpfs 126805 1 126804 1% /dev/shm
tmpfs 126805 3 126802 1% /run/lock
tmpfs 126805 16 126789 1% /sys/fs/cgroup
overlay 524288 524288 0 100% /var/lib/docker/overlay/acb2b31fc0e2944750c16050e035a9ecf31f824d8239cca227c99f256ce93d56/merged
shm 126811 1 126810 1% /var/lib/docker/containers/6b0603f536e33866ba51fb49fd2efc684e9f4c2297e6496bc25c4f243222e335/shm
tmpfs 126811 4 126807 1% /run/user/1000
Docker used up all inodes!
Delete the exited containers with their volumes:
docker rm -v <container_id>
And that freed up a bunch of inodes.
root@ip-172-31-51-118:~# df -i
Filesystem Inodes IUsed IFree IUse% Mounted on
udev 124890 393 124497 1% /dev
tmpfs 126805 523 126282 1% /run
/dev/xvda1 524288 524288 0 100% /
tmpfs 126805 1 126804 1% /dev/shm
tmpfs 126805 3 126802 1% /run/lock
tmpfs 126805 16 126789 1% /sys/fs/cgroup
overlay 524288 524288 0 100% /var/lib/docker/overlay/acb2b31fc0e2944750c16050e035a9ecf31f824d8239cca227c99f256ce93d56/merged
shm 126811 1 126810 1% /var/lib/docker/containers/6b0603f536e33866ba51fb49fd2efc684e9f4c2297e6496bc25c4f243222e335/shm
tmpfs 126811 4 126807 1% /run/user/1000