Docker volumes

As docker official gide says data volume is a directory within one or more containers that bypasses the UnionFS. With docker volumes you can share files between host and containers or even between different containers. They provide multiple useful features for persistent or shared data.

• Data volumes persist even if the container itself is deleted.
• Data volumes can be shared and reused between containers.
• Data volumes are initialized when the container is created and if the container image contains data at the specified mount point, that data is copied into the volume at the initialization of the volume.

How can I interact with a volume?

You can add a data volume to a container with the -v flag added to the docker create or docker run commands.

Note! To add multiple volumes to the container use -v multiple times.

$ docker run -d --name application -v /var/www iulyanp/alpine-php-fpm entrypoint.sh
15661fc3dbc7d93f9d6786ff05c5f640d52371d4670e2d48b88911b59d394763

This command will create /var/www folder inside our application container.

Note! You can also use the VOLUME instruction in a Dockerfile to add one or more new volumes to any container created from that image.

If we inspect the container we will see that the volume is mounted and has a hashed name. You will also see the Source which is the host location and the Destination which is telling us the volume location inside the container.

$ docker inspect application
...
"Mounts": [
   {
       "Name": "0e9f0824a292441e138aec50156c245fc69e7a4e697e20311b5376d6a37e6612",
       "Source": "/var/lib/docker/volumes/0e9f0824a292441e138aec50156c245fc69e7a4e697e20311b5376d6a37e6612/_data",
       "Destination": "/var/www",
       "Driver": "local",
       "Mode": "",
       "RW": true,
       "Propagation": ""
   },
]
...

If you’re using docker on linux you can also check to see what is in that volume by ls -la all the content from the Source, but as you can imagine nothing is there yet.

$ ls -la /var/lib/docker/volumes/0e9f0824a292441e138aec50156c245fc69e7a4e697e20311b5376d6a37e6612/_data
total 8
drwxr-xr-x 2 root root 4096 dec 17 14:03 .
drwxr-xr-x 3 root root 4096 dec 17 14:03 ..

Note! You should use sudo if you get Permission denied.

If you add something into the container at /var/www it will also appear in the Source.

$ docker exec -it application sh
/var/www # ls -la
total 8
drwxr-xr-x    2 root     root          4096 Dec 17 12:15 .
drwxr-xr-x   19 root     root          4096 Dec 17 12:15 ..
/var/www # touch index.html
/var/www # exit

$ sudo ls -al /var/lib/docker/volumes/0e9f0824a292441e138aec50156c245fc69e7a4e697e20311b5376d6a37e6612/_data
total 8
drwxr-xr-x 2 root root 4096 dec 18 14:35 .
drwxr-xr-x 3 root root 4096 dec 18 14:15 ..
-rw-r--r-- 1 root root    0 dec 18 14:35 index.html

Mount a host directory to the container

To mount an existing host directory into the container all we have to do is this.

$ docker run -d --name application -v /application:/var/www iulyanp/alpine-php-fpm entrypoint.sh

The command mounts the host directory /application into the container at /var/www. If the path already exists in the container image, the content inside it will not be removed, but the host directory will overlay the container one.

$ docker inspect application
...
"Mounts": [
    {
        "Source": "/application",
        "Destination": "/var/www",
        "Mode": "",
        "RW": true,
        "Propagation": "rprivate"
    },
]
...

$ docker exec -it application sh
/var/www # ls -la
total 216
drwxr-xr-x   12 1000     1000          4096 Dec 12 20:01 .
drwxr-xr-x   17 root     root          4096 Dec 12 20:23 ..
-rwxrwxrwx    1 1000     1000           533 Dec 12 20:01 index.php

This is how we use docker for development, we map the application folder into the docker container and any change we are doing on the host machine it will be visible into the container.

Named volumes

Named volumes are those volumes that you want to tag by name so you can use them later or when the container is deleted. To create a named volume just use the create command followed by the local driver and a specific name passed to the name option.

$ docker volume create --driver=local --name=application

If you want to see all docker volumes you have on your computer, list them with volume ls command.

$ docker volume ls

DRIVER              VOLUME NAME
local               application

When you need to remove a docker volume, the rm command comes to the rescue.

$ docker volume rm application

Sometimes you will see that there are volumes that you don’t use and they were never deleted. To delete those volumes you can use.

$ docker volume rm $(docker volume ls -f dangling=true -q)

To check where a volume is located you could once again use the inspect command to find out more about it.

$ docker volume inspect application

[
    {
        "Name": "application",
        "Driver": "local",
        "Mountpoint": "/var/lib/docker/volumes/application/_data",
        "Labels": {},
        "Scope": "local"
    }
]

$ sudo ls -la /var/lib/docker/volumes/application/_data

Conclusions

That was all for now. Go ahead and play around with volumes and became familiar with them. I’m sure you’ll find this feature very useful, and from now on you will be able to map data between different containers without any problem.

See you next time!

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