MariaDB Enterprise Server InnoDB Undo Log
The InnoDB undo log is a transaction log used by InnoDB to keep track of multiple row versions for multi-version concurrency control (MVCC). When a row's value changes, InnoDB stores old versions of the row in the Undo Log.
When transactions are committed and the old row versions are no longer necessary, the InnoDB Purge Threads asynchronously delete old row versions from the Undo Log in the background.
When a transaction is rolled back, InnoDB uses the Undo Log to rollback the transaction's changes.
InnoDB Undo Log
Multi-Version Concurrency Control (MVCC)
All ES and CS versions
Set by innodb_undo_tablespaces
10 MB per tablespace by default (grows as needed)
[mariadb] ... innodb_undo_tablespaces=8