Shared Volume

Saurabh Sharma

Volumes are managed by Docker and is the preferred way of persisting, sharing data.

Key Points

  • Volume drivers allows to
    1. Store volumes on remote hosts or cloud providers,
    2. Encrypt the contents of volumes.
  • Volumes does not increase the size of the containers using it.
  • Volume’s contents exist outside the lifecycle of a given container.
root@docker>docker volume ls
DRIVER              VOLUME NAME
local               c9ad85b13e22062f4eeefd9b65ae102c19856bea73622189f1a3393727c9b3e7

Options to define volumes

One an define a volume using either -v or --mount.

--mount

Option --mount is more explicit and allows one to avoid mistakes.

It consists of multiple key-value pairs, separated by commas and each consisting of a <key>=<value> tuple.

The option type can be either of

  1. volume
  2. bind
  3. tempfs

-v or --volume

The order of information provided is very important for this option.

-v or --volume Consists of three fields, separated by colon characters (:).

root@docker>docker volume

Usage:  docker volume COMMAND

Manage volumes

Commands:
  create      Create a volume
  inspect     Display detailed information on one or more volumes
  ls          List volumes
  prune       Remove all unused local volumes
  rm          Remove one or more volumes

Run 'docker volume COMMAND --help' for more information on a command.

Experiment time

You can simply create a volume without specifying any name

root@docker>docker volume create
502f4ffc940ead1fc8cfaed65a9829b17aac322f9b4125b57e3a2e66f113a658

docker volume ls will return the listing which will show the volume just created.

You can give name to volumes too.

root@docker>docker volume create my-volume
my-volume
root@docker>docker volume ls
DRIVER              VOLUME NAME
local               my-volume
root@docker>docker volume inspect my-volume
[
    {
        "CreatedAt": "2020-07-31T07:06:07Z",
        "Driver": "local",
        "Labels": {},
        "Mountpoint": "/var/lib/docker/volumes/my-volume/_data",
        "Name": "my-volume",
        "Options": {},
        "Scope": "local"
    }
]

deleting a volume is equally simple

root@docker>docker volume rm my-volume
my-volume

How about attaching a volume to a container?

Step 1: Create a volume

root@docker>docker volume create myvol
myvol
root@docker>docker volume ls
DRIVER              VOLUME NAME
local               myvol

Step 2: Refer to the volume

docker run -d --mount src=myvol,dst=/app --name my_nginx nginx:latest
Unable to find image 'nginx:latest' locally
latest: Pulling from library/nginx
6ec8c9369e08: Pull complete 
d3cb09a117e5: Pull complete 
7ef2f1459687: Pull complete 
e4d1bf8c9482: Pull complete 
795301d236d7: Pull complete 
Digest: sha256:0e188877aa60537d1a1c6484b8c3929cfe09988145327ee47e8e91ddf6f76f5c
Status: Downloaded newer image for nginx:latest
cee753ad7a48ccd4b1039391f7c10e8b710404472fce82e474319760e65bc89f

Inspecting the image can show the mount

root@docker>docker container inspect my_nginx
"Mounts": [
            {
                "Type": "volume",
                "Name": "myvol",
                "Source": "/var/lib/docker/volumes/myvol/_data",
                "Destination": "/app",
                "Driver": "local",
                "Mode": "z",
                "RW": true,
                "Propagation": ""
            }
        ]

If you look carefully there is a RW property which can be defined as false if you use the option ro, readonly in the option.

root@docker>ls -als /var/lib/docker/volumes/myvol/_data
total 0
0 drwxr-xr-x. 2 root root  6 Jul 31 07:12 .
0 drwxr-xr-x. 3 root root 19 Jul 31 07:12 ..

Inspecting the volume myvol

root@docker>docker inspect myvol
[
    {
        "CreatedAt": "2020-07-31T07:12:00Z",
        "Driver": "local",
        "Labels": {},
        "Mountpoint": "/var/lib/docker/volumes/myvol/_data",
        "Name": "myvol",
        "Options": {},
        "Scope": "local"
    }
]

The syntax in the --mount can be simplified by using --volume

docker run -d -v myvol:/app --name my_nginx nginx:latest

Please note if you are creating service you have to use the --mount option as the -v is not supported for docker service create

None of the containers can share the data if you use the local volume driver

References

  • https://docs.docker.com/storage/volumes/#share-data-among-machines

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