Managing the Redo log

Lesson 1

Redo logs are used by Oracle to preserve a record of transactions that have been issued against a database. In fact, transactions aren't considered to be committed until their changes have been written to the redo log. Once a record of a commit has been written to a redo log file, only then is a transaction secure.
This module will teach you how to manage and protect your redo log files. When you have finished this module, you should be able to:

1. Explain how to protect redo log files from loss and why it is important to do so
2. List the redo log files in your database
3. Create new redo log files for a database
4. Move redo log files
5. Drop redo log files
6. Manually force a log switch

Properly managing the database redo log is one of the most important responsibilities of a database administrator (DBA).

The redo log in Oracle Database plays a critical role in ensuring data integrity, durability, and recoverability of transactions. When a transaction is committed, the redo log records all changes made to the database, allowing Oracle to recover transactions in the event of a system failure. Here's a detailed explanation of the redo log's role during transaction commits:

1. Understanding the Redo Log: The redo log is a set of files on disk that record all changes made to the database's data blocks. It consists of:
   • Redo Log Buffer: An area in the system global area (SGA) memory where redo entries are temporarily stored.
   • Online Redo Log Files: Disk files where redo entries are permanently stored for recovery purposes.
2. Transaction Processing and Redo Entries: When a transaction modifies data (e.g., INSERT, UPDATE, DELETE operations), Oracle performs the following steps:
   • Generate Redo Entries: For every change, Oracle creates a redo record that describes how to reconstruct or "redo" the change.
   • Store in Redo Log Buffer: These redo records are placed into the redo log buffer in memory for efficiency.
3. The Role of Redo Log During Commit:
   When a transaction issues a COMMIT statement:
   1. Write Commit Record: Oracle generates a commit redo record and places it in the redo log buffer.
   2. Log Writer (LGWR) Activation: The Log Writer background process is signaled to write all redo entries, including the commit record, from the redo log buffer to the online redo log files on disk.
   3. Durability Assurance: Once LGWR confirms that the redo records are safely written to disk, Oracle acknowledges the commit to the user, ensuring the transaction is durable even in case of a failure.
   4. Release Locks: Oracle releases any locks held by the transaction, making changes visible to other users.
   5. Clean Up Resources: Transaction entries are removed from the undo segments if they are no longer needed for read consistency or rollback purposes.
   
   
4. Importance of Redo Logs in Recovery:
   • Instance Recovery: If the database instance crashes, upon restart, Oracle uses the redo logs to roll forward all committed transactions that were not yet written to the data files, ensuring no committed data is lost.
   • Media Recovery: In case of disk failures affecting data files, redo logs can be applied to a backup to recover the database to the point of failure.
5. Advantages of Using Redo Logs:
   • Data Integrity: Ensures that all committed transactions are preserved.
   • Performance Optimization: By writing sequentially to redo logs, Oracle minimizes disk I/O overhead.
   • Minimal Loss in Crashes: Enables recovery of transactions that might not have been saved to data files at the time of a crash.
6. Additional Considerations
   • Checkpoint Process: Periodically, Oracle writes dirty buffers from the database buffer cache to data files. However, this is independent of transaction commits.
   • Redo Log Configuration: Administrators can configure multiple redo log groups and members for redundancy and performance.
   • Archiving: In ARCHIVELOG mode, filled redo log files are archived before being reused, which is essential for point-in-time recovery.

Summary
The redo log ensures that all changes made by committed transactions are recorded and can be recovered in case of a system failure. During a commit:

• Redo entries are flushed from the redo log buffer to the online redo log files.
• This guarantees that the transaction's effects are durable.
• The redo logs serve as a reliable source for Oracle to reconstruct transactions during recovery operations.

Understanding the redo log's role is essential for database administrators to manage recovery procedures, optimize performance, and ensure data integrity within Oracle databases.
In the next lesson, you will learn how to and why you should protect your redo log files.