Redo Log vs. Undo Tablespace Architecture

Redo logs provide crash durability by replaying committed changes forward, while Undo tablespaces store pre-image rollback segments to service MVCC read consistency and rollbacks.

Internal Architecture Breakdown

Oracle separates forward recovery from historic version retention:

•Redo Log: Sequential append-only circular ring buffers replayed during instance startup to roll forward committed changes.
•Undo Tablespace: Maintains before-images of modified blocks, enabling multi-version point-in-time reads and clean statement-level rollbacks without locks.

Production Inspection Queries

Oracle

SELECT group#, thread#, sequence#, bytes/1024/1024 AS size_mb, status FROM v$log;
SELECT tablespace_name, status, retention FROM dba_tablespaces WHERE contents = 'UNDO';

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