How to change the data directory for MySQL or MariaDB in Ubuntu/Debian

Moving the data directory for MySQL or MariaDB frees space on a cramped root filesystem, enables faster storage, and can simplify backups by keeping database files on a dedicated volume.

Both MySQL and MariaDB read the data location from the server configuration (the datadir option in the [mysqld] section). The mysqld daemon opens system tables, redo logs, and per-database directories from that path during startup, so a move requires a clean shutdown and a faithful copy that preserves ownership and permissions.

On Ubuntu and Debian, AppArmor often confines mysqld to approved filesystem paths, so the profile needs an alias or explicit rules for the new directory or the service can fail with permission denials. A data directory located on a separate partition also needs a persistent mount that comes up before the database service to avoid starting against an unintended empty directory.

1. Record the current datadir value.

   $ sudo mysql --execute "SHOW VARIABLES LIKE 'datadir';"
   +---------------+----------------+
   | Variable_name | Value          |
   +---------------+----------------+
   | datadir       | /var/lib/mysql/|
   +---------------+----------------+

   Add --password when local authentication prompts for a password.

2. Stop the database service.

   $ sudo systemctl stop mysql

   Use

   $ sudo systemctl stop mariadb

   when the unit name is mariadb.service.

3. Create the new data directory with restrictive permissions.

   $ sudo install --directory --owner=mysql --group=mysql --mode=0750 /data/mysql

   The target filesystem needs POSIX permissions (for example, ext4 or xfs).

4. Confirm the new directory is on the intended filesystem.

   $ df -h /data/mysql
   Filesystem      Size  Used Avail Use% Mounted on
   /dev/sdb1       200G   20G  170G  11% /data

   A missing mount can hide the copied data later when the real filesystem mounts over the same path.

5. Copy the existing data files to the new directory with metadata preserved.

   $ sudo cp --archive /var/lib/mysql/. /data/mysql/

   Replace /var/lib/mysql with the path printed by the earlier datadir query when it differs.

6. Verify ownership and permissions on the new directory.

   $ sudo stat --format "%U:%G %a %n" /data/mysql
   mysql:mysql 750 /data/mysql

7. Rename the original data directory to keep a rollback copy.

   $ sudo mv /var/lib/mysql /var/lib/mysql.old

   Keep the original directory for rollback until service startup succeeds. Confirm the new datadir before deletion.

8. Open the server configuration file in an editor.

   $ sudo nano /etc/mysql/mysql.conf.d/mysqld.cnf

   MariaDB commonly uses /etc/mysql/mariadb.conf.d/50-server.cnf/ for server settings.

   Locate the active datadir setting with

   $ sudo grep --recursive --line-number "^[[:space:]]*datadir" /etc/mysql/

   .

9. Update the datadir setting under [mysqld] to the new path.

   [mysqld]
   datadir = /data/mysql

10. Open the AppArmor alias tunables file.

    $ sudo nano /etc/apparmor.d/tunables/alias

11. Add an alias mapping for the data directory.

    alias /var/lib/mysql/ -> /data/mysql/,

    When aliases are not used, add explicit rules to /etc/apparmor.d/usr.sbin.mysqld such as

    /data/mysql/ r,

    and

    /data/mysql/** rwk,

    .

12. Reload the mysqld AppArmor profile.

    $ sudo apparmor_parser --replace /etc/apparmor.d/usr.sbin.mysqld

13. Start the database service.

    $ sudo systemctl start mysql

    Use

    $ sudo systemctl start mariadb

    when the unit name is mariadb.service.

14. Confirm the service is running.

    $ sudo systemctl status mysql --no-pager
    ● mysql.service - MySQL Community Server
         Loaded: loaded (/lib/systemd/system/mysql.service; enabled; vendor preset: enabled)
         Active: active (running) since Fri 2025-12-12 09:41:04 UTC; 6s ago
       Main PID: 12345 (mysqld)
         Status: "Server is operational"
    ##### snipped #####

15. Verify datadir points at the new location.

    $ sudo mysql --execute "SHOW VARIABLES LIKE 'datadir';"
    +---------------+-------------+
    | Variable_name | Value       |
    +---------------+-------------+
    | datadir       | /data/mysql/|
    +---------------+-------------+

16. Inspect the journal for AppArmor denials when the service fails to start.

    $ sudo journalctl --unit=mysql --no-pager --since "10 minutes ago"
    ##### snipped #####
    apparmor="DENIED" operation="open" profile="usr.sbin.mysqld" name="/data/mysql/ibdata1" pid=12345 comm="mysqld"
    ##### snipped #####

    On MariaDB, replace mysql with mariadb in the unit name.

17. Remove the rollback directory when normal operation is confirmed.

    $ sudo rm --recursive --force /var/lib/mysql.old

    Deleting the rollback directory is irreversible, so keep a verified backup before running rm.