Persistent Storage

Container based file systems in docker are by default ephemeral. Volumes allow us to mount persistent disks to one or many containers.

Nginx example

$ docker run -d -p 8080:8080 -v volume_name:path_in_container <image_name>
$ docker run -p 8080:80 --name web --rm -v web:/usr/share/nginx/html nginx

To persist the volume on the host machine

$ docker run -d -p 8080:8080 -v host_path:path_in_container <image_name>
$ docker run -p 8080:80 --name web --rm -v $PWD:/usr/share/nginx/html nginx

Mounting manually

Creating a volume

$ docker volume create --name <volume_name>
$ docker volume create --name webdata

# Note: This volume can be later referenced by any container. Name helps finding it easily.

Attaching volume

# Run a daemon nginx container with webdata mounted
$ docker run -d --name web -v webdata:/usr/share/nginx/html -p 8000:80 nginx

# Check contents of default html
$ curl localhost:8000

Modify contents via docker

# By directly entering the container
$ docker exec web bash -c 'echo "foo" > /usr/share/nginx/html/index.html'

# Check contents of modified html
$ curl localhost:8000

Verify data persistence

# Stop and remove the web docker container
# Run a new container web1 using the same command

# Check contents of modified html
$ curl localhost:8000

# The changes made earlier should persist

Mounting the same volume to another container

# Run a new container web2 using the same command, except the port to be 8001

# Check contents of modified html
$ curl localhost:8001

# The changes made with container web1 should reflect from container web2 as well.

Inspecting the volume

$ docker inspect -f '{{ .Mounts }}' <container_name>
$ docker inspect -f '{{ .Mounts }}' web1

# -f flag is used to specify a format to filter results

# To find more information about the volume
$ docker volume inspect <volume_name>
$ docker volume inspect webdata

Other volume operations

# List all volumes
$ docker volume ls

# To remove a volume, all containers (where it is mounted on) must be stopped and removed
$ docker volume rm <volume_name>
$ docker volume rm webdata