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Table 5-1. Key Oracle Background Processes
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Database writer (DBWn) Log writer (LGWR)
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Writes modified data from the buffer cache to disk (datafiles) Writes redo log buffer contents to the online redo log files
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CH A PT ER 5 O RAC LE D AT ABA SE 11G A RCH I TEC TURE
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Table 5-1. Key Oracle Background Processes
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Checkpoint (CKPT) Process monitor (PMON) System monitor (SMON) Archiver (ARCn) Manageability monitor (MMON) Manageability monitor light (MMNL) Memory manager (MMAN) Job queue coordination process (CJQO)
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Updates the headers of all datafiles to record the checkpoint details Cleans up after finished and failed processes Performs crash recovery and coalesces extents Archives filled online redo log files Performs database-manageability-related tasks Performs tasks like capturing session history and metrics Coordinates the sizing of the SGA components Coordinates job queues to expedite job processes
I briefly discuss the main Oracle background processes in the following sections.
Database Writer
Oracle doesn t modify data directly on the disks all modifications of data take place in Oracle memory. The database writer process is then responsible for writing the dirty (modified) data from the memory areas known as database buffers to the actual datafiles on disk. It is the DBWn process s job to monitor the use of the database buffer cache, and if the free space in the database buffers is getting low, the database writer process makes room available by writing some of the data in the buffers to the disk files. The database writer process uses the least recently used (LRU) algorithm (or a modified version of it), which retains data in the memory buffers based on how long it has been since someone asked for that data. If a piece of data has been requested very recently, it s more likely to be retained in the memory buffers. The database writer process writes dirty buffers to disk under the following conditions: 1. When the database issues a checkpoint 2. When a server process can t find a clean reusable buffer after checking a threshold number of buffers 3. Every 3 seconds
Note When a user commits a transaction, it is not immediately made permanent by the database writer process with an immediate write to the database files. Oracle conserves physical I/O by waiting to perform a more efficient write of batches of committed transactions at once.
For very large databases or for databases performing intensive operations, a single database writer process may be inadequate to perform all the writing to the database files. Oracle provides for the use of multiple database writer processes to share heavy data modification workloads. You can have a maximum of 20 database writer processes (DBW0 through DBW9, and DBWa through DBWj). Oracle recommends using multiple database writer processes, provided you have multiple processors.
CHAPTER 5 ORAC LE DATABA SE 1 1G AR CHITECTURE
You can specify the additional database writer processes by using the DB_WRITER_PROCESSES initialization parameter in the SPFILE Oracle configuration file. If you don t specify this parameter, Oracle allocates the number of database writer processes based on the number of CPUs and processor groups on your server. For example, on my 32-processor HP-UX server, the default is four database writers (one database writer per eight processors), and in another 16-processor server, the default is two database writers. Oracle further recommends that you first ensure that your system is using asynchronous I/O before deploying additional database writer processes beyond the default number you may not need multiple database writer processes if so. (Even when a system is capable of asynchronous I/O, that feature may not be enabled.) If your database writer can t keep up with the amount of work even after asynchronous I/O is enabled, you should consider increasing the number of database writers.
Log Writer
The job of the log writer process is to transfer the contents of the redo log buffer to disk. Whenever you make a change to a database table (whether an insertion, update, or deletion), Oracle writes the committed and uncommitted changes to a redo log buffer (memory buffer). The LGWR process then transfers these changes from the redo log buffer to the redo log files on disk. The log writer writes a commit record to the redo log buffer and writes it to the redo log on disk immediately, whenever a user commits a transaction. If you ve multiplexed the redo log (as you re supposed to!), the log writer will write the contents of the redo log buffer to all members of the redo log group. If one or more members are damaged or otherwise unavailable, the log writer will just write to the available members of a group. If it can t write to even one member of a redo log group, the log writer signals an error. Each time the log writer writes to the redo log on disk, it transfers all the new redo log entries that arrived in the buffer since the log writer last copied the buffer contents to the redo log. The log writer writes all redo log buffer entries to the redo logs under the following circumstances: Every 3 seconds. When the redo log buffer is one-third full. When the database writer signals that redo records need to be written to disk. Under Oracle s write-ahead protocol, all redo records associated with changes in the block buffers must be written to disk (that is, to the redo log files on disk) before the datafiles on disk can be modified. While writing dirty buffers from the buffer cache to the storage disks, if the database writer discovers that certain redo information has not been written to the redo log files, it signals the log writer to first write that information, so it can write its own data to disk. In addition, as mentioned earlier, the log writer writes a commit record to the redo log following the committing of each transaction. The redo log files, as you learned earlier, are vital during the recovery of an Oracle database from a lost or damaged disk. Before the database writer writes the changed data to disk, it ensures that the log writer has already completed writing all redo records for the changed data from the log buffer to the redo logs on disk. This is called the write-ahead protocol. When you issue a commit statement to make your changes permanent, the log writer first places a commit record in the redo log buffer and immediately writes that record to the redo log, along with the redo entries pertaining to the committed transaction. The writing of the transaction s commit record to the redo log is the critical event that marks the committing of the transaction. Each committed transaction is assigned a system change number, which the log writer records in the redo
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