Difficulty: Medium
Correct Answer: Files that record all changes made to data blocks so that transactions can be recovered after a failure
Explanation:
Introduction / Context:
Redo logs are a fundamental component of the physical structure of Oracle style databases. They provide a chronological record of changes made to data blocks as transactions execute. This information is crucial for ensuring that committed transactions are not lost in the event of an instance failure or crash. Redo logs work in combination with datafiles, control files, and archived logs to support crash recovery and media recovery. Understanding what redo logs are and why they are essential is a key concept for database administrators and students of database systems.
Given Data / Assumptions:
Concept / Approach:
Redo logs record the before and after information needed to reconstruct changes to data blocks. When a user issues an insert, update, or delete, Oracle records the corresponding redo entries in memory and then writes them to redo log files on disk. In case of an instance failure, the database uses the redo logs during startup to roll forward any changes that were committed but not yet written to datafiles, ensuring transaction durability. For media recovery, archived copies of redo logs are applied after restoring backups to bring the database to a consistent point in time.
Step-by-Step Solution:
Step 1: Recognise that redo logs are not temporary; they are integral files that record all changes made to the database.Step 2: Understand that when transactions modify data, redo entries describing these modifications are generated and written to redo log buffers.Step 3: As the buffer fills or when a transaction commits, redo entries are flushed to online redo log files on disk.Step 4: During crash recovery, the database reads redo logs to identify committed changes that were not yet fully written to datafiles and applies them to ensure consistency.Step 5: In ARCHIVELOG mode, filled redo logs are archived, and these archived logs are later used in media recovery to roll forward a restored backup to a desired point in time.
Verification / Alternative check:
DBAs can observe the behaviour of redo logs through views that show log switches, archive status, and log file contents. When the database experiences a simulated crash, such as an abrupt shutdown, the next startup automatically performs instance recovery using redo logs, which is visible in the alert logs. Without redo logs, the database could not guarantee that committed transactions survive failures, and there would be no way to reconstruct consistent data from backups alone. This confirms their essential role in recovery and integrity.
Why Other Options Are Wrong:
Option B describes temporary files for query results, which are closer to temporary tablespace files or work areas, not redo logs. Option C refers to configuration files for user interfaces, which are outside the core database recovery mechanism. Option D talks about export and import archives, which are separate utilities for moving data between systems and do not replace redo logging. These alternatives do not match the definition and purpose of redo logs in Oracle style databases.
Common Pitfalls:
A common pitfall is placing all redo log members on a single disk, which can create a single point of failure. Best practice is to multiplex redo logs across different disks. Another mistake is not monitoring redo log size and frequency of log switches, which can affect performance and recovery times. DBAs should choose appropriate log sizes and ensure that archived logs are backed up and managed correctly. Understanding redo logs helps ensure that recovery procedures are reliable and that transactional integrity is maintained under all conditions.
Final Answer:
Correct answer: Files that record all changes made to data blocks so that transactions can be recovered after a failure
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