Support for read-only datadir mounts

[semohr]

Heya, maybe an easy question to answer. Is it possible to mount a data directory in read-only mode?

We have an archive drive on our cluster that is read-only by default, and I want to access data from an old MariaDB database occasionally. The database is multiple terabytes in scale so I want to prevent copying if possible. I tried testing this with Docker Compose, but ran into multiple issues:

1. Entrypoint chown : The official docker-entrypoint.sh tries to chown all files in the datadir, which fails on a read-only filesystem.
2. Server startup writes : Even bypassing the entrypoint, MariaDB tries to create test files( *.lower-test) and will abort with (Errcode: 30 "Read-only file system")

Minimal Compose example:

services:
  mariadb:
    image: mariadb:latest
    container_name: old_mariadb
    volumes:
      - ./data:/mnt/data:ro   # read-only dataset
    ports:
      - "3306:3306"
    entrypoint: ["mariadbd"]   # bypass entrypoint to skip chown
    command: >
      --datadir=/mnt/data
      --basedir=/usr
      --read-only=ON
      --skip-log-bin
      --tmpdir=/var/tmp

I thought this might be a good place to ask the question, has anyone run into the same issue? Any suggestions for running MariaDB directly on a read-only datadir would be highly appreciated. I might be missing something crucial here.

[ERROR] mariadbd: Can't create/write to file './ddl_recovery.log' (Errcode: 30 "Read-only file system")
[ERROR] DDL_LOG: Failed to create ddl log file: ./ddl_recovery.log
[ERROR] Aborting

[grooverdan]

@semohr, no you are not missing something crucial.

The opening of the ddl_recovery.log in read-only mode is definitely a mistake.

because read_only is a dynamic variable there needs to be some handling of setting read_only.

Please request this as a bug fix on https://jira.mariadb.org/

[semohr]

Thanks for the response!
I opened an issue here