MySQL 9.0 Release Notes
The data dictionary schema stores dictionary data in transactional
(InnoDB) tables. Data dictionary tables are
located in the mysql database together with
non-data dictionary system tables.
Data dictionary tables are created in a single
InnoDB tablespace named
mysql.ibd, which resides in the MySQL data
directory. The mysql.ibd tablespace file must
reside in the MySQL data directory and its name cannot be modified
or used by another tablespace.
Dictionary data is protected by the same commit, rollback, and
crash-recovery capabilities that protect user data that is stored
in InnoDB tables.