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Docker Volumes Explained: Keep Data When Containers Are Removed

Persist data with Docker volumes: create and attach them, share them between containers, keep a database's data, back them up and restore them, and clean up safely.

Intermediate5 min readUpdated
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A container's writable layer disappears with the container. A database that lost its data at every update would be useless, so Docker provides volumes: storage that lives outside the container.

The problem#

Terminal
docker run --name c1 alpine:3.24 sh -c 'echo "important" > /data.txt'
docker rm c1 > /dev/null
docker run --rm alpine:3.24 cat /data.txt
Output
cat: can't open '/data.txt': No such file or directory

Use a volume#

-v name:/path attaches a named volume at that path in the container. Docker creates the volume if needed:

Terminal
docker run --rm -v notes:/data alpine:3.24 sh -c 'echo "important" > /data/note.txt'
docker run --rm -v notes:/data alpine:3.24 cat /data/note.txt
Output
important

The first container is gone, but the data is still there.

Manage volumes#

Terminal
docker volume ls --format '{{.Name}} {{.Driver}}'
docker volume inspect notes --format '{{.Mountpoint}}'
Output
notes local
/var/lib/docker/volumes/notes/_data

Docker stores volumes in its own area on the host. You rarely need to look inside it.

Share a volume#

Several containers can use one volume at the same time:

Terminal
docker run -d --name writer -v shared:/data alpine:3.24 sh -c 'echo "from writer" > /data/msg; sleep 30' > /dev/null
sleep 1
docker run --rm -v shared:/data:ro alpine:3.24 cat /data/msg
docker rm -f writer > /dev/null
Output
from writer

:ro mounts it read-only.

A real database#

The official PostgreSQL image stores its data in /var/lib/postgresql/data. Attach a volume there:

Terminal
docker run -d --name db -e POSTGRES_PASSWORD=devpass -v pgdata:/var/lib/postgresql/data postgres:17-alpine > /dev/null
until docker exec db pg_isready -q; do sleep 1; done
sleep 2
docker exec db psql -U postgres -qc "CREATE TABLE notes (body text); INSERT INTO notes VALUES ('Saved in a volume');"
docker rm -f db > /dev/null
docker run -d --name db -e POSTGRES_PASSWORD=devpass -v pgdata:/var/lib/postgresql/data postgres:17-alpine > /dev/null
until docker exec db pg_isready -q; do sleep 1; done
sleep 1
docker exec db psql -U postgres -tAc "SELECT body FROM notes;"
docker rm -f db > /dev/null
Output
Saved in a volume

We deleted the container and made a new one, and the data survived. This is how you upgrade a database container safely.

Back up a volume#

There is no special command. Run a short-lived container with the volume, and a folder for the archive:

Terminal
mkdir backup
docker run --rm -v notes:/data:ro -v "$PWD/backup":/backup alpine:3.24 tar -czf /backup/notes.tar.gz -C /data .
ls backup
docker volume create restored > /dev/null
docker run --rm -v restored:/data -v "$PWD/backup":/backup alpine:3.24 tar -xzf /backup/notes.tar.gz -C /data
docker run --rm -v restored:/data alpine:3.24 cat /data/note.txt
Output
notes.tar.gz
important

For databases, use their own tool, such as pg_dump, because copying the files of a running database may not give a consistent backup.

Clean up#

Terminal
docker volume rm notes restored
docker volume prune -af > /dev/null
docker volume ls -q | wc -l
Output
notes
restored
0

docker volume prune deletes volumes not used by a container. Read the list carefully on a real server.

Common mistakes#

Practice#

Practice exercise

Create a volume called counter. Run a container three times, each adding one line to /data/visits.txt, then print the number of lines.

Show solution
Terminal
for i in 1 2 3; do
  docker run --rm -v counter:/data alpine:3.24 sh -c 'echo visit >> /data/visits.txt'
done
docker run --rm -v counter:/data alpine:3.24 wc -l /data/visits.txt
Output
3 /data/visits.txt

Key takeaway#

Next: Bind Mounts.

Finished this lesson?

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