====== Docker ======

**General Information**

Installing and configuring docker.

  * **Docker Versions Change Log** - Change branches to see different change logs
    * 17.12: https://github.com/docker/docker-ce/blob/17.12/CHANGELOG.md

**Checklist**
  * CentOS 7

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====== Docker: Upgrade to CE ======

Docker has switched from docker-engine packages to docker-ce (community edition) and docker-ee (enterprise edition).

docker-engine-ce's last version was 17.05.

Official documentation: https://docs.docker.com/engine/installation/linux/docker-ce/centos/

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====== Docker: Install ======

  * Ensure system is updated<code bash>sudo yum update</code>
  * Add the Docker repo
    * Repo<code bash>yum-config-manager --add-repo https://download.docker.com/linux/centos/docker-ce.repo</code>
    * Add the GPG key<code bash>rpm --import https://download.docker.com/linux/centos/gpg</code>
  * Install Docker
    * CE (community edition)<code bash>yum install docker-ce</code>

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====== Docker: Storage ======

**Docker Data and Meta Data**
  * Create a "thin" logical volume using all of the space on a physical volume<code bash>sudo lvcreate --extents 100%PVS --thin --name lvdockerpool vglocal /dev/sdX1</code>
    * This thin logical volume will be used by the docker daemon directly to automatically setup a file system and allocate space for docker data and metadata.
    * **NOTE:** This space is NOT used for docker container image storage. (/var is the default)

\\
If you ever need to increase docker container data storage OR metadata storage, they can be resized individually.
  * Resize docker pool metadata example (increase by 1 GB)<code bash>lvextend --size +1G -n vglocal/lvdockerpool_tmeta</code>
  * Resize docker pool storage example (increase by 10 GB)<code bash>lvextend --size +10G -n vglocal/lvdockerpool_tdata</code>

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**Docker container image storage (/var)**
    * If /var needs more space for container image storage, run the following (example)<code bash>sudo lvextend --resizefs --size +7G /dev/vglocal/lvvar</code>

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====== Docker: Service ======

**Service Setup**
  * Override default docker.service startup options
    * Create a systemd docker.service override directory
<code bash>sudo mkdir /etc/systemd/system/docker.service.d</code>
    * Create a docker.service override file
      * **Docker 1.12 (2016-07-28) - 17.12.0-ce (2017-12)**<code bash>tee /etc/systemd/system/docker.service.d/docker.conf <<-'EOF'
[Service]
EnvironmentFile=
EnvironmentFile=-/etc/sysconfig/docker
ExecStart=
ExecStart=/usr/bin/dockerd $OPTIONS
EOF</code>
      * The blank variable setting ensures that no default settings are appended to the overrides.

    * Setup docker daemon startup options
      * **Docker 1.12 (2016-07-28) - 17.12.0-ce (2017-12)**<code bash>tee /etc/sysconfig/docker <<-'EOF'
# Modify these options if you want to change the way the docker daemon runs
OPTIONS=--storage-driver=devicemapper --storage-opt dm.thinpooldev=vglocal-lvdockerpool --storage-opt dm.use_deferred_deletion=true --storage-opt dm.use_deferred_removal=true
EOF
</code>
    * Reload the systemd daemon for the override file to be picked up<code bash>systemctl daemon-reload</code>

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====== Docker: Start and Test ======

  * Start the Docker engine, enable on boot, and check status<code bash>sudo systemctl start docker
sudo systemctl enable docker
sudo systemctl status docker</code>
  * Verify docker was started with the correct options<code bash>ps -elf | grep docker | grep -v grep</code>
  * Run the test image to ensure it is installed correctly<code bash>sudo docker run hello-world</code>
  * Create the "docker" group and add users to it<code bash>sudo usermod -aG docker <username></code>
    * Multiple users example<code bash>for x in user1 user2 user3; do sudo usermod -aG docker $x; done</code>
  * Log out/log back in
  * Verify the user can run docker commands without using sudo<code bash>docker run hello-world</code>

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====== Troubleshooting ======

  * **Symptom**: Docker service fails to start
  * **Error Seen In Logs**: Error starting daemon: error initializing graphdriver: devicemapper: Non existing device vglocal-lvdockerpool
  * **Why**: /var/lib/docker contains metadata telling docker about the contents of device mapper storage. When /var/lib/docker is removed, metadata is lost. Then docker will detect the thin pool has data, but docker is unable to use it.
  * **Official Documentation**: https://access.redhat.com/solutions/2281281 

__**The Fix**__

  * Stop docker<code bash>systemctl stop docker</code>
  * Remove the docker thin pool logical volume<code bash>lvremove /dev/vglocal/lvdockerpool</code>
  * Re-create it (example using 100% of /dev/sde1)<code bash>lvcreate --extents 100%PVS --thin --name lvdockerpool vglocal /dev/sde1</code>

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