Volumes and Bind Mounts

everything is isolated from the host, including the filesystem, you can think of Docker containers as mini virtual machines. Whenever you re-create a container, the old one is entirely destroyed, including its filesystem.

Volumes are paths on the host mounted inside the container, allowing for persistent storage. Removing a container does not remove the files in the volume, as they are only mounted to the container upon its startup.

There are three types of volumes:

  • Named volumes
    • they are managed by Docker and created automatically in its storage location, usually /var/lib/docker/volumes
  • Unnamed volumes
    • these act just like named volumes, but they get a randomly generated name
  • Bind mounts
    • used to mount specific paths from the host inside a container

All volumes are technically mounts, the difference is that volumes are managed by docker. You can have a volume with a specific path, but that’s more typing for really the same effect. I don’t really see a reason not to use bind mounts.

Syntax

Either in the docker run command, in a compose file, or in a container manager such as Portainer, volumes are usually declared as:

volumes:
  - /path/on/host:/path/inside/container

On the left side, you will set the path to the directory or file on the host system, and on the right side is the path the directory/file will be inside the container.

REMEMBER

Left = path on the host/on your machine/server/computer

Right = path inside the container


For example, if you want to store your WordPress installation in /mnt/disk1, you would use the following volume syntax:

volumes:
  - /mnt/disk1/wordpress:/var/www/html # Mounting directories
  - /mnt/disk1/config/wp-config.php:/var/www/html/wp-config.php # Mounting files

The mount order is from top to bottom, so even if there’s already a wp-config.php in your /mnt/disk1/wordpress directory, the one from /mnt/disk1/config will be inside the container.

Read-only

There are some files which you might not want the container to modify. In that case, you can set the volume as read-only by adding “:ro” to the end of it

volumes:
  - /mnt/disk1/wordpress:/var/www/html 
  - /mnt/disk1/config/wp-config.php:/var/www/html/wp-config.php:ro

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