Problem

Using a data container to store the MySQL data does not work properly if I mount the parent directory /var/lib instead of /var/lib/mysql

Infrastructure

I'm using 3 docker containers to provide a website:

• nginx container to run a webserver


• mariadb container for the database


• data container for the created user data

The data container is created using the following docker command:

$ docker create 

     -v /var/lib 

     -v /var/www 

     --name [appname]-data 

     tianon/true /bin/true

Checking the available mounts for [appname]-data with $ docker inspect [appname]-data:

"Mounts": [

        {

            "Name": "361ed7b3e38371653f2df75652973e1c48db52d59c4c5371f634c1930291af0c",

            "Source": "/var/lib/docker/volumes/361ed7b3e38371653f2df75652973e1c48db52d59c4c5371f634c1930291af0c/_data",

            "Destination": "/var/lib",

            "Driver": "local",

            "Mode": "",

            "RW": true,

            "Propagation": ""

        },

        ...

The containers are started with docker-compose. Both containers should use all mounts available on the data container. My docker-compose.yml:

[appname]-nginx:

  container_name: [appname]-nginx

  image: nginx:9-apache

  links:

    - [appname]-mariadb:mysql

  volumes_from:

    - [appname]-data

  restart: always

  expose:

    - "80"



[appname]-mariadb:

  image: mariadb:10.1

  container_name: [appname]-mariadb

  restart: always

  volumes_from:

    - [appname]-data

  environment:

    MYSQL_ROOT_PASSWORD: pas$w0rd

    TERM: dumb

The output of $ docker exec -it [appname]-nginx ls -lha /var/lib/mysql:

total 8.0K

drwxr-xr-x 2 root root 4.0K Jun 27 16:30 .

drwxr-xr-x 3 root root 4.0K Jul 21 10:19 ..

The output of $ docker exec -it [appname]-mariadb ls -lha /var/lib/mysql:

total 109M

drwxr-xr-x 4 mysql mysql 4.0K Jul 21 10:19 .

drwxr-xr-x 3 root  root  4.0K Jul 21 10:19 ..

-rw-rw---- 1 mysql mysql  16K Jul 21 07:57 aria_log.00000001

-rw-rw---- 1 mysql mysql   52 Jul 21 07:57 aria_log_control

-rw-rw---- 1 mysql mysql  48M Jul 21 07:57 ib_logfile0

-rw-rw---- 1 mysql mysql  48M Jul 21 07:56 ib_logfile1

-rw-rw---- 1 mysql mysql  12M Jul 21 07:57 ibdata1

-rw-rw---- 1 mysql mysql    0 Jul 21 07:56 multi-master.info

drwx------ 2 mysql mysql 4.0K Jul 21 07:57 mysql

drwx------ 2 mysql mysql 4.0K Jul 21 07:56 performance_schema

-rw-rw---- 1 mysql mysql  24K Jul 21 07:57 tc.log

Executing mount on both containers shows the same:

/dev/dm-0 on /var/lib type ext4 (rw,relatime,data=ordered)

/dev/dm-0 on /var/www type ext4 (rw,relatime,data=ordered)

However for [appname]-mariadb has an additional mount for /var/lib/mysql exists:

/dev/dm-0 on /var/lib/mysql type ext4 (rw,relatime,data=ordered)

Question

Why is it not possible to create a volume mount on /var/lib to save all stored data in the data container?

Do I miss something?

Is it a problem of MySQL / MariaDB?

Why don't you create your docker volume from docker-compose.yml as well?
The following should work:

version: '2'                       



services:                          

  nginx:                           

    image: nginx:9-apache          

    expose:                        

      - '80'                       

    volumes:                       

      - app-data:/var/www          

  mariadb:                         

    image: mariadb:10.1            

    volumes:                       

      - app-data:/var/lib          

    environment:                   

      MYSQL_ROOT_PASSWORD: pas$w0rd

      TERM: dumb                   



volumes:                           

  app-data:

I wonder why you'd want both containers to store their data on the same volume though. I'd imagine it preferable to store nginx's and mariadb's data in separate volumes.