Difference Between Redo Log and Archive Log in Oracle Database
In Oracle Database architecture, Redo Logs and Archive Logs play a critical role in data protection and recovery.
Many beginners and even some experienced DBAs often confuse these two concepts because both are related to database changes.
However, their purpose, behavior, and usage are very different.
In this blog, we will clearly explain:
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What Redo Logs are
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What Archive Logs are
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Key differences between them
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How they work internally
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Real-world DBA use cases
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Interview-ready comparison
What is a Redo Log?
A Redo Log is a file that records every change made to the Oracle database, whether the transaction is committed or not.
Whenever a user performs an operation like:
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INSERT
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UPDATE
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DELETE
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DDL statements
Oracle records the change in the Redo Log Buffer, and then LGWR (Log Writer process) writes it to the redo log files.
Key Characteristics of Redo Logs
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Stores change vectors (not actual data)
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Written sequentially
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Required for instance recovery
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Works in a circular fashion
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Overwritten automatically
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Mandatory for all Oracle databases
Redo Log Architecture
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Redo Log Buffer (SGA)
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Online Redo Log Files
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Log Groups and Members
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LGWR Process
Why Redo Logs Are Important
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Crash recovery
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Instance recovery
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Protects against memory failures
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Ensures database consistency
Without redo logs, Oracle cannot recover from crashes.
What is an Archive Log?
An Archive Log is a copy of a filled redo log file that is saved to a separate location before the redo log is reused.
Archive logs are generated only when the database is running in ARCHIVELOG mode.
The ARCH (Archiver) process is responsible for copying filled redo logs to archive destinations.
Key Characteristics of Archive Logs
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Stores historical redo data
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Created after redo log switch
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Used for media recovery
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Not overwritten automatically
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Required for point-in-time recovery
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Mandatory for Data Guard & Standby databases
Archive Log Architecture
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ARCH Process
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Archive Destinations
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Archive Log Files
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FRA (Fast Recovery Area)
Redo Log vs Archive Log – Key Differences
| Feature | Redo Log | Archive Log |
|---|---|---|
| Purpose | Records live database changes | Stores historical redo data |
| Written by | LGWR | ARCH |
| Mode dependency | Required in all modes | Only in ARCHIVELOG mode |
| Overwritten | Yes (circular) | No |
| Used for | Instance recovery | Media & PITR recovery |
| Data retention | Short-term | Long-term |
| Required for Data Guard | Indirectly | Yes |
How Redo and Archive Logs Work Together
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User performs a transaction
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Changes go to Redo Log Buffer
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LGWR writes changes to Redo Log File
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Redo Log becomes full
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Log switch occurs
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ARCH copies redo log to archive location
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Redo log is reused
This mechanism ensures zero data loss recovery capability when properly configured.
Real-World DBA Scenarios
Scenario 1: Database Crash
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Oracle uses Redo Logs to recover uncommitted transactions
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Archive logs are not involved
Scenario 2: Disk Failure
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Datafiles are restored from backup
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Archive logs are applied
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Database recovers to the latest point
Scenario 3: Data Guard Standby
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Archive logs are shipped to standby
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Applied for synchronization
Scenario 4: Point-in-Time Recovery
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RMAN uses archive logs
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Restores database to a specific SCN or time
What Happens If ARCHIVELOG Mode Is Disabled?
❌ No point-in-time recovery
❌ No Data Guard
❌ Risk of data loss
❌ Limited backup strategy
✔ Only suitable for test or non-critical environments
DBA Best Practices
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Always enable ARCHIVELOG mode in production
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Monitor archive log space (FRA)
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Implement regular archive log backups
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Delete archive logs after backup using RMAN
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Multiplex redo logs for high availability
Interview Perspective (Must-Know)
Simple way to explain in interviews:
Redo Logs capture live database changes, while Archive Logs store those changes permanently for recovery.
Conclusion
Understanding the difference between Redo Logs and Archive Logs is foundational for every Oracle DBA.
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Redo Logs ensure database consistency
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Archive Logs ensure data recoverability
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Together, they protect Oracle databases from failures and disasters
If you are working in production support, Data Guard, RMAN, or HA environments, mastering this topic is non-negotiable.
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