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Archived Redo LogEdit

Archived Redo Log

Archived redo logs are a fundamental mechanism for data durability in databases that use a redo logging approach. In systems such as Oracle Database, every change made to the data files is recorded in a sequential log. When the active redo log becomes full, the database writes a copy of that log to a separate archival location. Those archived copies, known as Archived Redo Logs, can then be used to reconstruct the database state in the event of a failure, enabling recoveries to a precise point in time or to a specific SCN. This archiving process is controlled by the database operating mode and by the configuration of the archiving destinations, and it is typically managed by dedicated background processes that ensure the integrity and availability of the archived data.

Overview

  • Archived redo logs are the durable record of all committed changes that have not yet been flushed to the data files. They complement the online redo logs, which are used for ongoing recovery within a single instance.
  • The ability to archive redo logs requires the database to be in ARCHIVELOG mode, which guarantees that a copy of every completed redo record is persisted somewhere other than the active online redo log.
  • Archived logs are typically consumed by backup and recovery processes, such as RMAN, to restore databases after a crash or to perform point-in-time recovery.

How archived redo logs work

  • As changes are made, the online redo log captures redo entries. When a log file fills, the log writer triggers a switch that makes room for new entries.
  • In ARCHIVELOG mode, an Archiver process copies the completed redo log to an archival destination. This destination can be local storage, a remote server, or a cloud-based location, depending on configuration.
  • Each archived log is associated with a log sequence number and a thread, and the files are named and cataloged so that recovery can locate the exact files needed for a given point in time.
  • The archived logs are essential for media recovery, as well as for restoring a database to a specific moment after an outage or corruption.

Configuring and managing archived redo logs

  • Enabling archiving requires setting the database to ARCHIVELOG mode, which provides the mechanism for preserving redo data beyond the current online logs.
  • Archiving destinations are configured with parameters that designate where archived files should be written and how to handle failures (for example, when a destination is temporarily unavailable).
  • It is common to use multiple destinations or to configure offsite or remote archiving to protect against site failures. This improves disaster recovery capabilities but introduces considerations for latency, bandwidth, and consistency.
  • Logs must be managed to avoid running out of space. Monitoring and automated cleanup policies, in combination with backup strategies, help maintain a reliable archive set.

Backup, recovery, and point-in-time recovery

  • Archived redo logs play a central role in backup strategies. They enable RMAN-based backups to be recoverable to a precise moment or SCN, even after the database files have been damaged.
  • Point-in-time recovery (PITR) uses archived logs to roll the database forward from a known consistent state to a designated target time, capturing changes that occurred after the last backup.
  • If archived logs are missing or unavailable, recovery options become severely limited, underscoring the importance of robust storage, validation, and offsite replication for archived logs.
  • RMAN can leverage archived redo logs to perform incremental restorations and to maintain a recoverable history for audits or compliance requirements.

Performance and storage considerations

  • Archiving adds I/O load and requires sufficient disk or network resources to copy filled redo logs promptly. Delays in archiving can create pressure on the online redo log, potentially impacting transaction throughput.
  • Organizations often dedicate separate storage for archived redo logs and implement monitoring to prevent archiving destinations from filling up, which could stall database operations.
  • A balance exists between immediate archiving performance and long-term recoverability. Some environments trade off shorter retain cycles for simplicity, while others emphasize long-term retention for stringent recovery objectives.

Security, integrity, and compliance

  • Archived redo logs contain a complete transcript of changes and, as such, are high-value data from a security and compliance standpoint. Access controls, encryption at rest, and integrity checks help protect archived logs from tampering or unauthorized access.
  • Regular verification of the archival process, along with integrity checksums and redundant storage, improves confidence that recoveries will be possible when needed.
  • Regulatory regimes in some industries may dictate specific retention periods and access controls for archival data, influencing how archived redo logs are stored and managed.

Controversies and debates (practical, technical perspectives)

  • One common debate centers on archiving reliability versus performance. Some environments favor aggressive offloading of archived logs to remote or cloud storage to improve local disk performance, while others push for local, fast-access archives to minimize recovery time if a disk failure occurs.
  • Another discussion concerns retention policies. Longer retention improves disaster recovery options but increases storage costs and management complexity. Organizations must weigh risk, regulatory requirements, and budget when determining how long to keep archived logs.
  • There is also consideration of default configurations versus customized architectures. Some practitioners advocate for standard, out-of-the-box defaults to reduce configuration errors, while others argue for tailored architectures that align with organizational DR plans and recovery objectives.
  • In debates about backup windows and maintenance, the timing of backups relative to peak load can influence archiving strategies. Critics of aggressive archiving schedules may worry about increased operational overhead, while proponents highlight the value of ensuring recoverability across failures.

See also