Samarthya

The storage driver allows your to create data in the writable layers of the container and it also controls how images and containers are stored and managed on the Docker host.

The files will not be persisted after the container is deleted, and performance wise it is relatively slower than native fs performance.

E.g. Let’s run a image hello-world

docker run --name hello hello-world

Hello from Docker!
This message shows that your installation appears to be working correctly.

To generate this message, Docker took the following steps:
 1. The Docker client contacted the Docker daemon.
 2. The Docker daemon pulled the "hello-world" image from the Docker Hub.
    (amd64)
 3. The Docker daemon created a new container from that image which runs the
    executable that produces the output you are currently reading.
 4. The Docker daemon streamed that output to the Docker client, which sent it
    to your terminal.

To try something more ambitious, you can run an Ubuntu container with:
 $ docker run -it ubuntu bash

Share images, automate workflows, and more with a free Docker ID:
 https://hub.docker.com/

For more examples and ideas, visit:
 https://docs.docker.com/get-started/

Let’s inspect the container, and look for GraphDriver

docker container inspect hello

        "GraphDriver": {
            "Data": {
                "LowerDir": "/var/lib/docker/overlay2/228075bcce28b817db0857971a8da5209a5b4743fd2c2a462243bd8d793555ff-init/diff:/var/lib/docker/overlay2/39cd21c862ae937b5ac61b392f85dc65c04c7e0cabbf7a15408bf5cc14bd2232/diff",
                "MergedDir": "/var/lib/docker/overlay2/228075bcce28b817db0857971a8da5209a5b4743fd2c2a462243bd8d793555ff/merged",
                "UpperDir": "/var/lib/docker/overlay2/228075bcce28b817db0857971a8da5209a5b4743fd2c2a462243bd8d793555ff/diff",
                "WorkDir": "/var/lib/docker/overlay2/228075bcce28b817db0857971a8da5209a5b4743fd2c2a462243bd8d793555ff/work"
            },
            "Name": "overlay2"
        },

You can see the Name of the driver and some locations mentioned in Data

"WorkDir": "/var/lib/docker/overlay2/228075bcce28b817db0857971a8da5209a5b4743fd2c2a462243bd8d793555ff/work"

If I do a listing in my docker host system I can see this directory

ls -als /var/lib/docker/overlay2/228075bcce28b817db0857971a8da5209a5b4743fd2c2a462243bd8d793555ff/work
total 0
0 drwx------. 3 root root 18 Jul 30 09:32 .
0 drwx------. 4 root root 55 Jul 30 09:32 ..
0 d---------. 2 root root  6 Jul 30 09:32 work

How about the image?

docker image inspect hello-world

Look for GraphDriver again

"GraphDriver": {
            "Data": {
                "MergedDir": "/var/lib/docker/overlay2/39cd21c862ae937b5ac61b392f85dc65c04c7e0cabbf7a15408bf5cc14bd2232/merged",
                "UpperDir": "/var/lib/docker/overlay2/39cd21c862ae937b5ac61b392f85dc65c04c7e0cabbf7a15408bf5cc14bd2232/diff",
                "WorkDir": "/var/lib/docker/overlay2/39cd21c862ae937b5ac61b392f85dc65c04c7e0cabbf7a15408bf5cc14bd2232/work"
            },
            "Name": "overlay2"
        },

A quick directory listing can reveal the contents

ls -als /var/lib/docker/overlay2/39cd21c862ae937b5ac61b392f85dc65c04c7e0cabbf7a15408bf5cc14bd2232

total 12
0 drwx------.  3 root root   47 Jul 28 08:08 .
8 drwx------. 50 root root 4096 Jul 30 09:32 ..
0 -rw-------.  1 root root    0 Jul 30 09:32 committed
0 drwxr-xr-x.  2 root root   19 Jul 28 08:08 diff
4 -rw-r--r--.  1 root root   26 Jul 28 08:08 link

Types of drivers

• overlay2
• aufs
• devicemapper
• btrfs
• vfs

Read more here.

• overlay2 is the preferred storage driver for all currently supported Linux distributions.
• aufs is preferred for the kernels with no support for overlay2 for Ubuntu 14.01 and older.
• devicemapper requires direct-lvm and is recommended storage driver for CentOS7 and earlier and RHEL. (The recent versions do support overlay2)

Supported backing filesystems

Data persistence is managed using several storage models

1. Filesystem storage
   • Used by overlay2 and aufs, and is memory efficient.
2. Block storage
   • devicemapper uses it, and is useful for write heavy workloads.
3. Object storage
   • It is highly flexible and scalable object based store, e.g. A rest API that can push and pull data.

Checking the storage driver

You can use docker info to check the driver on your localhost.

 Server Version: 19.03.12
 Storage Driver: overlay2
  Backing Filesystem: xfs
  Supports d_type: true
  Native Overlay Diff: true

References

• https://docs.docker.com/storage/storagedriver/select-storage-driver/#docker-engine—community
• https://rancher.com/block-object-file-storage-containers/
• https://docs.docker.com/storage/bind-mounts/
• https://docs.docker.com/storage/volumes/