In Oracle Database, what is redo log file mirroring and why is it used for protecting redo information?

Introduction / Context:
This question examines your knowledge of Oracle recovery mechanisms, specifically redo log file mirroring. Redo logs capture all changes made to the database and are critical for instance recovery and media recovery. Losing redo information can result in serious data loss. Redo log mirroring, sometimes called multiplexing, is one of the techniques used to increase the reliability of redo storage.

Given Data / Assumptions:

- We are discussing Oracle online redo log files, not archived logs or standby databases.
- The goal of mirroring is to protect redo information from disk failures.
- The question focuses on what mirroring is, not the detailed configuration steps.

Concept / Approach:
Redo log file mirroring in Oracle is implemented by creating multiple members for each redo log group. Each member is a separate physical file, ideally placed on a different disk. All members of a group are written to simultaneously. If one member is lost due to disk failure, the database can still function using the remaining member, thereby preventing loss of redo data. This is a basic form of redundancy at the redo log level.

Step-by-Step Solution:
Step 1: Recall that redo log files are grouped, and each group can have multiple members. Step 2: Understand that mirroring or multiplexing means having more than one physical copy of each group on different disks. Step 3: Review option A and confirm that it precisely describes multiple copies of each redo log file on different disks to protect against media failure. Step 4: Check option B, which refers to copying archived logs to tape, a separate archiving or backup process. Step 5: Observe that option C talks about compression, which is not the main idea of mirroring and does not address redundancy. Step 6: Recognize that option D describes database replication or Data Guard style standby databases, which is a different high level disaster recovery feature.

Verification / Alternative check:
To verify, you can inspect the V$LOGFILE view in Oracle, which lists all redo log members and their group numbers. When there are multiple members for a group, mirroring is in effect. Oracle documentation also uses the term multiplexing for this feature and explains that it protects against the loss of redo log data due to disk issues. This confirms that option A is aligned with Oracle practice.

Why Other Options Are Wrong:
Option B deals with archiving and backup, which are important but not the same as mirroring active redo logs. Option C focuses on compression, which may be used in storage systems but is not the core definition of mirroring. Option D describes logical or physical standby databases that replicate whole database changes to another site. While that also protects against data loss, it is not what the term redo log file mirroring refers to in Oracle terminology.

Common Pitfalls:
A common pitfall is assuming that archiving or backups alone are sufficient to protect redo logs while the database is running. Without mirroring, the loss of a single, active redo log file can still cause recovery problems. Another pitfall is placing mirrored log members on the same disk or in the same directory, which defeats the purpose of redundancy. Correct configuration should distribute them across different physical devices.

Final Answer:
Redo log file mirroring is the practice of keeping multiple copies of each redo log file on different disks to protect redo information from media failure.