The Oracle Processes Layer
The Oracle Processes layer uses an OraProcessTest to track the health of the individual processes corresponding to the Oracle database server. The key processes associated with an Oracle database server are:
- The system monitor process (smon): The usage and function of this Oracle background process is twofold. First, in the event of an instance failure—when the memory structures and processes that comprise the Oracle instance cannot continue to run—the smon process handles recovery from that instance failure. Second, the smon process handles disk space management issues on the database by taking smaller fragments of space and “coalescing” them, or piecing them together.
- The process monitor process (pmon): This process watches the user processes on the database to make sure that they work correctly. If for any reason a user process fails during its connection to Oracle, pmon will clean up the remnants of its activities and make sure that any changes it may have made to the system are “rolled back,” or backed out of the database and reverted to their original form.
- The log writer process (lgwr): This background process handles the writing of redo log entries from the redo log buffer to online redo log files on disk.
- The database writer process (dbw): This background process handles all data block writes to disk. Working in conjunction with the Oracle database buffer cache memory structure, this process prevents users from ever accessing a disk to perform a data change such as update, insert, or delete.
- The checkpoint process (ckpt): This process is used to handle writing log sequence numbers to the datafile headers and control file, alleviating the log writer process of that responsibility.
The recoverer process (reco): This background process handles the resolution of distributed transactions against the database.
Figure 1 : The tests associated with the Oracle Processes layer