Volumes

What is a volume?

A volume is a storage area that Docker manages for you, living outside any single container. You attach a volume to a container, and anything written there is saved in the volume - not in the container. Remove the container, and the volume (and its data) stays. Attach it to a new container, and the data is right there.

Volumes are the recommended way to store data that must survive, like databases.

Create and use a Docker volume

You can create a volume explicitly:

docker volume create mydata

Then attach it to a container with -v, giving the volume name and the path inside the container where it should appear:

docker run -it -v mydata:/root ubuntu bash

The -v mydata:/root part means "mount the mydata volume at /root inside the container". Now anything written to /root goes into the volume. Try it:

echo "this will survive" > /root/notes.txt
exit

Start a brand-new container with the same volume:

docker run -it -v mydata:/root ubuntu bash
cat /root/notes.txt

This time the file is there. The data lived in the volume, so the new container sees it.

A real example: a database

This is exactly how you keep a database's data safe. For example, running MySQL and storing its data in a volume:

docker run -d --name db \
  -e MYSQL_ROOT_PASSWORD=secret \
  -v db-data:/var/lib/mysql \
  mysql:8

MySQL stores its files in /var/lib/mysql inside the container, and we've pointed that at the db-data volume. Now you can stop and remove the container, start a new one with the same volume, and all your tables and rows are still there.

Manage volumes with docker volume

List your volumes:

docker volume ls

Remove one you no longer need (this deletes its data, so be careful):

docker volume rm mydata

Volumes are perfect for data Docker should manage. But during development you often want to share your own project folder with a container - that's what bind mounts are for, coming up next.

A safety habit: named volumes over anonymous ones

When you attach a volume, give it a name (-v db-data:/var/lib/mysql) rather than letting Docker create an anonymous one. Anonymous volumes get random IDs, are easy to lose track of, and pile up as junk you can't identify later. A named volume is easy to find with docker volume ls, easy to reuse, and easy to back up. Future-you will thank present-you.

FAQ

What is the difference between a volume and a bind mount?

A volume is storage Docker manages internally - you refer to it by name and don't pick a folder on your disk. A bind mount points at a specific folder on your computer. Volumes suit persistent app data; bind mounts suit sharing source code in development (next lesson).

Where does Docker store volume data?

In an area Docker manages on the host (you don't normally browse it directly). That's the point - you interact with the volume by name via docker volume, and Docker handles where the bytes live.

Does removing a container delete its volume?

No. That's the whole benefit - the volume and its data outlive the container. A volume is only deleted when you remove it explicitly with docker volume rm.