DBWn, LGWR, SMON

DBWn, LGWR, and SMON DBWn of the oracle 10g process: Database writer Databasewriter in order to ensure that the buffer zone has free buffer for users. DBWn applies the content of the buffer zone to the data file and writes the DOS command for data modification in the database buffer pool to the disk. The db_writer_process parameter can specify the number of DBWn processes. The maximum value of this parameter is 20. If db_writer_process is not specified when the database is started, oracle automatically determines the size of db_writer_process Based on the cpu and processor of the computer. In oracle 10 Gb, the following conditions will trigger the DBWn process: 1. The system does not have any extra blank buffer to store data; 2. the CKPT process triggers the DBWn process. LGWR: log writerprocess log writer this process is responsible for managing the log buffer pool and writing data in the redo log buffer pool to the redo log file on the disk. In the following cases, the LGWR process will execute: 1. When the user commits a transaction; 2. every three seconds; 3. When the redo log buffer uses 1/3, and there is 1 MB of dirty data; 4. When the DBWn process writes the modified DOS command to the disk and needs to redo the log. Note the following: 1. Before DBWn writes the modified DOS command, the redo log records associated with the changes in the buffer zone must be written to the disk. If DBWn finds that some redo records are not written to the disk, it will prompt LGWR to write these redo logs to the disk and wait until LGWR completes before writing data to the disk. 2. The LGWR process will be triggered when the transaction is committed, but sometimes a large buffer is required. At this time, LGWR will write the transaction into the redo log before the transaction is committed, these log records are permanently stored in the redo log records when the transaction is committed. Each time a user commits a transaction, the transaction will be assigned an SCN (System Change number), which will be recorded in the redo log file together with the transaction redo log by oracle. Record the scn in the redo log file to ensure that the actual application cluster and distributed database can be synchronized during the recovery operation. The log buffer is a circular buffer. When LGWR writes logs in the log buffer to the log file, the server process can write new logs to the log buffer. LGWR is usually written very quickly to ensure that there is always space in the log buffer to write new logs. SMON: systemmonitor process system monitoring process the system monitoring process recovers when the instance is started. System Monitoring is responsible for clearing the temporary segments in the temporary tablespace when the database is started, or for some temporary segments left over during abnormal operations. For example, when an index is created, the segment assigned to the index during creation is marked as Temporary. If the create index (or rebuild index) Session is interrupted due to exceptions for some reasons, smon is responsible for clearing these temporary segments. When the pctomcrease of a tablespace is set to a non-zero value, smon is responsible for combining the extent that is idle in the tablespace and adjacent to each other into a large idle expansion zone.