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Lesson 5 Database redo log files
ObjectiveDiscuss the role of redo log files.

Database Redo Log Files in Oracle 13c: A Comprehensive Overview

Oracle Database 13c, a sophisticated and powerful database management system, is designed to deliver high performance, reliability, and scalability to support enterprise applications and data warehouses. One of the critical components in ensuring the integrity and recoverability of the database is the use of redo log files. These files play a crucial role in Oracle 13c's database architecture, safeguarding data and enabling recovery in case of failures or data corruption.

1) The Redo Log Mechanism

The redo log mechanism is an essential aspect of Oracle 13c, responsible for recording changes made to the database. These changes, known as redo entries, are generated whenever a transaction modifies data within the database. The primary purpose of redo log files is to capture and store these redo entries, thereby enabling the database to recover from failures, including instance and media failures.

2) Components of Redo Log Files

Oracle 13c uses a combination of online and archived redo log files:
  1. Online Redo Log Files: Oracle 13c maintains a set of two or more online redo log files, which are organized into groups. These files are written to in a cyclical fashion, and each group has a unique log sequence number. Oracle 13c's online redo log is a crucial component of the system as it ensures that committed data is durable, even in the event of an instance failure.
  2. Archived Redo Log Files: Once a redo log group becomes inactive, its contents can be archived to create an archived redo log file. This process is necessary for maintaining a backup of the online redo log, and it plays a vital role in database recovery scenarios. Archived redo log files are particularly useful for point-in-time recovery or during the backup and recovery process of a database.

3) Functions of Redo Log Files in Oracle 13c

Redo log files in Oracle 13c serve several important functions:
  1. Data Recovery: Redo log files are indispensable during database recovery operations. They contain the necessary information to reconstruct changes made to the database since the last backup. In the event of a failure, Oracle 13c uses redo log files to apply changes and restore the database to a consistent state.
  2. Instance Recovery: If an instance fails, the online redo log files allow Oracle 13c to perform an automatic instance recovery during startup. This process ensures the database's consistency and durability by rolling forward transactions that were committed but not yet written to the data files and rolling back uncommitted transactions.
  3. Media Recovery: In the event of media failure, such as a damaged data file, Oracle 13c uses archived redo log files to perform media recovery. By applying the redo log files to the most recent backup, the database can be restored to the point of failure or a specified point in time.
  4. Oracle Data Guard and Standby Databases: Redo log files play a critical role in maintaining standby databases, which are used for disaster recovery and high availability. Oracle Data Guard, a key feature of Oracle 13c, relies on redo log files to keep the standby databases synchronized with the primary database.

In conclusion, database redo log files in Oracle 13c are indispensable components that ensure data integrity, durability, and recoverability. By maintaining and managing online and archived redo log files, Oracle 13c can efficiently handle failures, recover lost data, and support high-availability solutions.

Role of Database redo log files

Redo log files are used to record all changes or transactions made to the database. Every Oracle database has a minimum of two online redo log files. The primary purpose of online redo logs is to protect the database from an instance failure by storing before and after images of modified data. As users generate transactions, changes to data are stored first in memory. These changes are then written to the redo log buffer and LGWR writes this information to a redo log file. Note that LGWR is the only process that can write to redo log files. Eventually, the DBWR process writes the modified data stored in the disk buffer cache to the physical database files.
This allows the instance processes to efficiently manage writing information to disk. Writing to disk is much slower than writing to memory. By writing information to the redo logs first, Oracle does not need to constantly write data to the data files. This provides for an overall improved performance.
If you create a default database using the Oracle Database Assistant, you have four redo log files created for you. If your ORACLE_HOME is c:\oracle8, the redo log files are located in c:\oracle8\database. If your database SID is orc1, the redo log files are named log1orc1.ora, log2orc1.ora, log3orc1.ora, and log4orc1.ora.
Having all your redo log files in one subdirectory on one disk is not a good situation. If the disk crashes, the data on the redo logs is lost. Oracle provides a way to minimize this situation with multiplexed redo log file groups, which will be covered shortly.
The next lesson is about writing data to the redo log files.