Oracle RAC Reference
The Oracle database is very rich in terms of diagnostic data and statistics. It provides an enormous amount of statistics about most of the internal operations, which are exposed through a group of underlying views maintained by the database server. These views are normally accessible to the database administrator user SYS. They are called dynamic performance views because they are continuously updated while a database is open and in use, and their contents relate primarily to performance.
These views are built on the set of internal memory structures commonly known as x$views. These internal and undocumented views ...