Difficulty: Medium
Correct Answer: It allows complete and point in time recovery using archived redo logs after media failures
Explanation:
Introduction / Context:
Many enterprise databases, such as Oracle, support two basic modes for handling redo log files: ARCHIVELOG and NOARCHIVELOG. These modes directly affect backup and recovery strategies. In ARCHIVELOG mode, filled redo log groups are archived before being reused, preserving a complete record of all changes. In NOARCHIVELOG mode, redo logs can be reused without being archived, which limits recovery options. Understanding why ARCHIVELOG mode is preferred for production systems is a key topic for database administrators and is often tested in interviews.
Given Data / Assumptions:
Concept / Approach:
The main advantage of ARCHIVELOG mode is that it enables complete recovery and point in time recovery even after media failures that damage data files. By keeping archived copies of filled redo logs, the database can be restored from a previous backup and then rolled forward using archived logs to reapply all changes. This is critical for systems that require high availability and minimal data loss. In NOARCHIVELOG mode, redo logs are reused without being archived, restricting recovery to the time of the last cold or consistent backup and increasing the risk of data loss if a failure occurs after that backup.
Step-by-Step Solution:
Step 1: Recognise that redo logs capture all changes to the database, including insert, update, and delete operations.Step 2: In ARCHIVELOG mode, when a redo log group fills up, the database automatically copies it to an archive location before reusing it.Step 3: During recovery, an administrator can restore data files from a backup and then apply archived redo logs in sequence to bring the database to a desired point in time.Step 4: This allows recovery after media failures, such as disk crashes, with minimal or no data loss depending on backup frequency and archive retention.Step 5: Compare this with NOARCHIVELOG mode, where filled redo logs are overwritten without archiving, making it impossible to roll forward beyond the last consistent backup.
Verification / Alternative check:
In a lab environment, you can create a test database in ARCHIVELOG mode, perform a full backup, make additional data changes, and then simulate a failure by deleting a data file. By restoring the backup and applying archived redo logs, you can recover the database to a time just before the failure. If you repeat the experiment in NOARCHIVELOG mode, once a failure occurs after the last backup, you cannot apply any additional changes because the necessary redo information has not been archived. This practical difference demonstrates why ARCHIVELOG mode provides stronger recovery options.
Why Other Options Are Wrong:
Option B states that ARCHIVELOG mode disables redo logging, which is the opposite of reality; redo logging remains active and its contents are archived. Option C claims that ARCHIVELOG mode prevents users from performing data changes, which would defeat the purpose of a production system. Option D suggests that ARCHIVELOG is required only for temporary databases, whereas in practice serious production databases almost always use ARCHIVELOG, and temporary or test systems may run in NOARCHIVELOG mode when data loss is acceptable.
Common Pitfalls:
A common pitfall is enabling ARCHIVELOG mode without planning sufficient storage for archived logs, which can lead to full archive destinations and halted database operations. Another mistake is failing to test recovery procedures, leading to unpleasant surprises during real incidents. Administrators should design a backup and archive strategy that includes regular full and incremental backups, monitoring of archive destinations, and periodic recovery drills. When used correctly, ARCHIVELOG mode is a powerful foundation for robust database recovery and high availability.
Final Answer:
Correct answer: It allows complete and point in time recovery using archived redo logs after media failures
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