InnoDB data structures
In MariaDB, by default, InnoDB is mapped to XtraDB, a compatible InnoDB fork maintained by Percona.
InnoDB tables are contained in tablespaces. A tablespace is a file that contains data and indexes for one or more tables. In old MariaDB and MySQL versions, all the tables are created in a system tablespace. If the innodb_file_per_table system variable is set to 1, which is the default since the 10.0 Version, each table is stored in a separate tablespace. This variable is dynamic, so it is possible to store some tables in separate files, and others in the system tablespace.
The system tablespace, by default, also contains InnoDB's data dictionary, the undo logs, the change buffer, and the doublewrite buffer. The data dictionary is a metadata collection of all InnoDB tables, columns, and indexes. The system tablespace is stored in the data directory, in the ibdata files (by default, two files).
A portion of a tablespace is called a segment. Regular tablespaces have one segment for data and one segment for each index. The system tablespace has several segments.
A page is a small data unit stored in a tablespace or in the buffer pool. Pages have a fixed size that can be configured. A page contains one or two rows and usually some empty space. The non-empty space ratio is called the fill factor.
A page that has been modified in the change buffer is called a dirty page.
In some cases, for example for a consistent read process, InnoDB sequentially reads several pages together, with a total size of 1 MB. Such groups of pages are called extents.
InnoDB indexes are important not only for reads, but also for locks. Each lock points to an index record.
An InnoDB index can be a clustered index or a secondary index. Primary keys are clustered indexes. If a table does not have a primary key, the first UNIQUE index, which only contains NOT NULL columns, will be used as a primary key. If no such index exists, a hidden cluster index is automatically created. All secondary index records point to a clustered index record, so we can say that all secondary indexes contain the clustered index.
