[docker] Understanding "VOLUME" instruction in DockerFile

To better understand the volume instruction in dockerfile, let us learn the typical volume usage in mysql official docker file implementation.

VOLUME /var/lib/mysql

Reference: https://github.com/docker-library/mysql/blob/3362baccb4352bcf0022014f67c1ec7e6808b8c5/8.0/Dockerfile

The /var/lib/mysql is the default location of MySQL that store data files.

When you run test container for test purpose only, you may not specify its mounting point,e.g.

docker run mysql:8

then the mysql container instance will use the default mount path which is specified by the volume instruction in dockerfile. the volumes is created with a very long ID-like name inside the Docker root, this is called "unnamed" or "anonymous" volume. In the folder of underlying host system /var/lib/docker/volumes.

/var/lib/docker/volumes/320752e0e70d1590e905b02d484c22689e69adcbd764a69e39b17bc330b984e4

This is very convenient for quick test purposes without the need to specify the mounting point, but still can get best performance by using Volume for data store, not the container layer.

For a formal use, you will need to specify the mount path by using named volume or bind mount, e.g.

docker run  -v /my/own/datadir:/var/lib/mysql mysql:8

The command mounts the /my/own/datadir directory from the underlying host system as /var/lib/mysql inside the container.The data directory /my/own/datadir won't be automatically deleted, even the container is deleted.

Usage of the mysql official image (Please check the "Where to Store Data" section):

Reference: https://hub.docker.com/_/mysql/

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