What is "xtradb"

Percona XtraDB is a storage engine for the MariaDB and Percona Server databases, and is intended as a drop-in replacement to InnoDB, which is one of the default engines available on the MySQL database.

XtraDB incorporates InnoDB's ACID-compliant design and MVCC architecture, and allows for a greater degree of tuning and scalability. The engine is also better suited to multi-core processing, which addresses some of the issues known to InnoDB.