E.6 Restrictions on XA Transactions  XA transaction support is
limited to the InnoDB storage engine.  For external XA, a MySQL
server acts as a Resource Manager and client programs act as
Transaction Managers. For Internal XA, storage engines within a
MySQL server act as RMs, and the server itself acts as a TM.
Internal XA support is limited by the capabilities of individual
storage engines. Internal XA is required for handling XA
transactions that involve more than one storage engine. The
implementation of internal XA requires that a storage engine
support two-phase commit at the table handler level, and
currently this is true only for InnoDB.   For XA END, the
SUSPEND [FOR MIGRATE] clause is not supported.  The requirement
that the bqual part of the xid value be different for each XA
transaction within a global transaction is a limitation of the
current MySQL XA implementation. It is not part of the XA
specification.