In MySQL 8 installations on Ubuntu, both directories serve as include locations for configuration files:

/etc/mysql/conf.d/      # For user/administrator configurations
/etc/mysql/mysql.conf.d/ # For package-maintained configurations

The main distinction lies in their management:

• mysql.conf.d/: Maintained by MySQL packages (deb/rpm). Contains critical files:

  mysqld.cnf    # Main server configuration
  mysql.cnf     # Client configuration
  mysqldump.cnf # Dump utility configuration

• conf.d/: Intended for custom configurations. Empty by default.

Files are loaded alphabetically from both directories. Later settings override earlier ones. Example loading sequence:

1. /etc/mysql/mysql.conf.d/mysqld.cnf
2. /etc/mysql/conf.d/custom.cnf

For custom configurations:

1. Always use /etc/mysql/conf.d/ for your changes
2. Create separate files for logical groupings:

   # Recommended naming convention
   custom-optimization.cnf
   custom-replication.cnf

3. Never modify files in mysql.conf.d/ directly

Instead of editing mysqld.cnf, create:

# /etc/mysql/conf.d/buffer-pool.cnf
[mysqld]
innodb_buffer_pool_size = 4G

To verify loaded configurations:

mysql --help --verbose | grep -A 10 "Default options"
# Or check the effective configuration:
SELECT * FROM performance_schema.variables_by_thread
WHERE VARIABLE_NAME LIKE '%buffer%';

The separation serves important purposes:

• Prevents package upgrades from overwriting custom configs
• Maintains clean distinction between system and user settings
• Enables safer configuration management

When working with MySQL 8 on Ubuntu, you'll notice two configuration directories:

!includedir /etc/mysql/conf.d/
!includedir /etc/mysql/mysql.conf.d/

The mysql.conf.d/ directory is typically managed by the MySQL package maintainers. This is where the distribution stores its default configuration files, including:

mysql.cnf
mysqld.cnf

On the other hand, conf.d/ is designed for user-specific configurations and is the recommended location for your custom configurations.

For your own configuration files, you should use /etc/mysql/conf.d/. This ensures:

• Your configurations won't be overwritten during package upgrades
• Clear separation between system and custom configurations
• Better maintainability when troubleshooting

MySQL processes files in alphanumeric order within each directory. Files in mysql.conf.d/ typically load first (starting with 'm'), followed by conf.d/ files. You can verify this with:

mysql --help | grep "Default options"

To add a custom buffer pool size configuration:

# Create a new config file in the proper directory
sudo nano /etc/mysql/conf.d/custom.cnf

# Add your custom settings
[mysqld]
innodb_buffer_pool_size = 4G

# Restart MySQL to apply changes
sudo systemctl restart mysql

If you encounter problems, check the loading order and potential conflicts:

sudo mysqld --verbose --help | grep -A 1 "Default options"

Remember that later configurations override earlier ones with the same parameters.