Docker Volume

 

  • Volumes are directories (or files) that are outside of the default Union File System and exist as normal directories and files on the host filesystem.

Why do we need volume?

  • To understand why we need volumes lets go back to simple image busybox.

  • docker run -it busybox

    • Create a directory and add a file. touch test.txt
    • Run new container with same image, list the files and you won't see the file you created.
    • Why not run same container? If we run same container we will still see that file.
    • Remember container are ephemeral and disposable.
    • We should not depend on starting same container.

  • How do we persist changes, so that when we run new container we see that file?

Named Volume

  • The most direct way is declare a volume at run-time with the -v @name:@path flag.
  • @name is name of the volume.
  • @path is the path inside container.

-v @name:@path

docker run -v myVolume:/my-volume --it busybox
  • This will make the directory /my-volume inside the container live outside the UFS and directly accessible on the host.
  • Any files that the image held inside the /my-volume directory will be copied into the volume.
  • If the container path specified doesn't exist docker will create one for you.

Where can I find the file in host system?

  • We can see where docker is storing our file by inspecting the container.
  • docker inspect @container_id
  • If you see "Mounts" property, you will see properties like
    • "Source" is path in the host
    • "Destination" is path in the container
  • If you try to navigate to path specified by "Source" in you computer, you won't find it. Why?
  • For docker that host path is inside Virtual Machine.
  • We can also see docker volume list to list all volumes.
  • As you can see volume name is big hash at the moment

Manually create a named volume

  • To manually create a named volume, you can use docker volume create command.
docker volume create --name @volume\_name

Reattaching same volume

  • If you attach a same volume when running new container, you will find all previous files.
docker run -v myVolume:/my-volume -it busybox

# Create a file inside my-volume directory
> touch my-volume/test.txt

# List content of my-volume directory
> ls my-volume/
test.txt
  • Running new container with same volume
docker run -v myVolume:/my-volume -it busybox
# List content of my-volume directory
> ls my-volume/
test.txt