Generation of IO operations in Oracle

I/O includes two parts, read and write, to introduce the generation of write operations in Oracle databases first.

2.1. Write

Before introducing write operations, simply look at the physical structure of Oracle: Oracle's physical files include the following three files: control files, redo log files (Redo log file), data Files (datafiles). In the data file, according to the different functions, can also be divided into: System data files, user data files, temporary space files and rollback section files. Also, if the Archive log mode of the database is activated, there is an archive log file. Oracle's I/O generation is the data read and write operations on these files. The following is a detailed look at the production of several major writing operations and their processes.

2.1.1. control files

The physical structure information of the entire database is recorded in the control file. At the same time the control file also records the system and the various data files of the SCN (systems change number, about the SCN can be seen in the article "Orac le SCN mechanism detailed") information for data recovery, so the data file on the SCN changes, the Oracle database will also be The SCN information on the control file should be modified.

2.1.2. Redo Log

In the case of direct write, write operations in a transaction produce Redo log, which is used as a recovery record when a block of data is abnormally closed. Similarly, the Redo log is not written directly to the Redo log file, but is written to the log Buffer, similar to the write user data.

Log buffer is a buffer that can be reused. The LGWR process is responsible for writing the records in log Buffer to Redo log File.

Once the entries in log Buffer are written to the Redo log file, they can be reused.

In order to secure the log BUFFER,LGWR process as quickly as possible, the data in log Buffer is usually written to the Redo log file.

In the following situations, LGWR writes a sequential log Buffer to the Redo log file:

A. When a transaction is committed (a commit).

B. Write Log Buffer every 3 seconds.

C. When Log Buffer is full 1/3.

D. When the DBWN process writes "Dirty" data to disk.

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