For the first record section of the controlfile, the database information entry section, this requirement is trivial, because the database information entry only takes about 210 bytes and is therefore guaranteed to always fit into a single controlfile block that can be written atomically. Therefore changes to the database entry can be implicitly committed as they are written, without any recoverability concerns.
Recoverability for changes to the other controlfile records sections is provided by maintaining all the information in duplicate. Each logical block is represented by two physical blocks. One contains the current information, and the other contains either an old copy of the information, or a pending version that is yet to be committed. To keep track of which physical copy of each logical block contains the current information, Oracle maintains a block version bitmap with the database information entry in the first record section of the controlfile.
To read information from the controlfile, a session must first read the block version bitmap to determine which physical block to read. Then if a change must be made to the logical block, the change is first written to the alternate physical block for that logical block, and then committed by atomically rewriting the block containing the block version bitmap with the bit representing that logical block flipped. When changes need to be made to multiple records in the same controlfile block, such as when updating the checkpoint SCN in all online datafiles, those changes are buffered and then written together. Note that each controlfile transaction requires at least 4 serial I/O operations against the controlfile, and possibly more if multiple blocks are affected, or if the controlfile is multiplexed and asynchronous I/O is not available. So controlfile transactions are potentially expensive in terms of I/O latency.
Whenever a controlfile transaction is committed, the controlfile sequence number is incremented. This number is recorded with the block version bitmap and database information entry in the first record section of the controlfile. It is used in the cache header of each controlfile block in place of an SCN to detect possible split blocks from hot backups. It is also used in queries that perform multiple controlfile reads to ensure that a consistent snapshot of the controlfile has been seen. If not, an ORA-00235 error is returned.
The controlfile transaction mechanism is not used for updates to the checkpoint heartbeat. Instead the size of the checkpoint progress record is overstated as half of the available space in a controlfile block, so that one physical block is allocated to the checkpoint progress record section per thread. Then, instead of using pairs of physical blocks to represent each logical block, each checkpoint progress record is maintained in its own physical block so that checkpoint heartbeat writes can be performed and committed atomically without affecting any other data.
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