Data persistence

Learn to keep data even after a container is removed.

Data persistence

A container's writable layer is removed together with the container. When you delete and recreate a database container to update it, all data is lost. Docker offers two ways for persistent data:

Volume: storage managed by Docker (/var/lib/docker/volumes).
Bind mount: mounting a specific folder on the host into the container.

docker run -d --name db -v pgdata:/var/lib/postgresql/data postgres:16

A life independent of the container

A volume isn't deleted when the container is removed. When a new container mounts the same volume, it continues from where the data left off. That makes an image update as simple as "remove and restart with the new version":

docker rm -f db
docker run -d --name db -v pgdata:/var/lib/postgresql/data postgres:16.4
# tables and records are in place

Exercise A persistent database

Start a container named db from postgres:16 in the background with POSTGRES_PASSWORD=gizli; mount the pgdata volume at /var/lib/postgresql/data.

Test yourself

What happens if a database container is removed and recreated without a volume?
- The data is backed up automatically
- Nothing changes
- All data is lost
- The data is saved into the image
Show answer

All data is lost — The data was in the removed container's writable layer.

Mount the pgdata volume at the data directory.

docker run -d ____ pgdata:/var/lib/postgresql/data postgres:16

Show answer

docker run -d -v pgdata:/var/lib/postgresql/data postgres:16

Match.

writable layer

Removed with the container
volume

Managed by Docker, persistent
bind mount

Host folder
/var/lib/docker/volumes

Where volumes live

Next lesson: Named volumes

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