My favourite, TokuDB has online DDL. Which is a welcome change to the MySQL ecosystem.

I am pretty sure the only other engines that support online DDL is Sybase and DB2.

Online DDL generally means lock free DDL operations.

Not even Oracle, MSSQL, or Postgresql do online DDL. But at least they have transactional DDL, even if it isn't entirely lock-free all the time.

Compared to InnoDB/XtraDB; TokuDB lacks foreign key support, because this was worked into InnoDB years ago because the MySQL server layer doesn't support any kind of constraints. I am hoping for MariaDB to start implementing more modern SQL features which would this be available in supported storage engines.