In the XA environment, each database is considered a separate resource manager. When a transaction manager wants to access two databases under the same transaction, it must use the XA protocols to perform two-phase commit with the two resource managers.
Because each independent disk pool is a separate SQL database, in the XA environment each independent disk pool is also considered a separate resource manager. For an application server to perform a transaction which targets two different independent disk pools, the transaction manager must also use a two-phase commit protocol.