Volumes and Bind Mounts: Keeping Your Data

Introduction

Containers are meant to be disposable: you remove them and start new ones whenever you update an image. But databases, uploaded files and logs must survive that. Docker solves this by storing such data outside the container, in volumes or bind mounts.

Container Data Is Temporary

Each container has its own writable layer on top of the image. Priya writes a file in a container, removes the container, then looks for the file in a new one:

docker run --name temp alpine sh -c "echo important > /data.txt"
docker rm temp
docker run --rm alpine cat /data.txt

The file died with the container. The new container started fresh from the image, which never contained /data.txt.

Named Volumes

A named volume is storage managed by Docker that exists independently of any container. Create one, then mount it into a container with -v VOLUME:PATH:

docker volume create notes-data

Three separate containers now use the same volume — two each add a line, the third reads the file:

docker run --rm -v notes-data:/data alpine sh -c 'echo "first note" >> /data/notes.txt'
docker run --rm -v notes-data:/data alpine sh -c 'echo "second note" >> /data/notes.txt'
docker run --rm -v notes-data:/data alpine cat /data/notes.txt

Every container was removed straight away (--rm), yet both notes survived, because they were written to the volume rather than to a container’s own layer. (If you mount a volume name that does not exist yet, Docker creates it automatically.)

Bind Mounts

A bind mount shares a folder from your own computer with a container — ideal during development, because changes appear immediately. Priya has a one-line website in ~/site/index.html and serves it with Nginx by mounting the folder over Nginx’s web root. The host path must be absolute, so $PWD (the current directory) is used:

docker run -d --name site -p 8080:80 -v "$PWD/site":/usr/share/nginx/html:ro nginx:alpine > /dev/null
sleep 1
curl -s http://localhost:8080/

She edits the file on her computer — no rebuild, no restart — and the container serves the new version at once:

echo "<h1>Priya's site, version 2</h1>" > site/index.html
curl -s http://localhost:8080/

Read-Only Mounts

The :ro at the end of Priya’s bind mount made it read-only inside the container. Nginx only needs to read the website, so even if the web server were compromised, it could not change the files:

docker exec site sh -c "echo hacked > /usr/share/nginx/html/index.html"

A Database That Keeps Its Data

The most important use of volumes is databases. Priya runs PostgreSQL 18 with a volume called pgdata for its data directory. (The official PostgreSQL 18 image keeps its data under /var/lib/postgresql. -e sets an environment variable — the image requires a password; lesson 9 covers this.)

docker run -d --name db -e POSTGRES_PASSWORD=secret -v pgdata:/var/lib/postgresql postgres:18-alpine

After a few seconds the database is ready. She creates a table and adds two rows with psql, PostgreSQL’s command-line client, which is included in the image:

docker exec db psql -U postgres -c "CREATE TABLE notes (id serial PRIMARY KEY, text text);" -c "INSERT INTO notes (text) VALUES ('Buy milk'), ('Learn Docker volumes');"

Now the real test — she deletes the database container completely:

docker rm -f db

She starts a brand-new container with the same docker run command (same volume), waits for it to be ready, and queries the table:

docker exec db psql -U postgres -c "SELECT * FROM notes;"

The data is all there. This is exactly how you upgrade a database container in practice: remove the old container, start one from the newer image with the same volume.

Managing Volumes

docker volume ls

(The listing was taken before PostgreSQL was started, so only notes-data appears.) docker volume inspect shows where Docker keeps a volume on the host:

docker volume inspect notes-data

Docker will not remove a volume that a container is using:

docker volume rm pgdata

After removing the containers, Priya deletes the notes volume by name, then removes every volume no container uses with docker volume prune -a:

docker rm -f db site > /dev/null
docker volume rm notes-data
docker volume prune -a -f

Common Mistakes

Frequently Asked Questions

Key Takeaways

Summary

Your containers can now keep their data safely. Next you will configure containers without rebuilding them, using environment variables — and learn how to handle passwords properly.