Undo Log
A log structure retaining prior state needed to roll back changes or expose an earlier row version.
Undo information is not limited to rolling back a failed transaction. In MVCC systems, readers may also depend on retained prior versions, so long-running transactions can affect the lifetime and cleanup pressure of undo data.
Redo and undo serve different recovery directions: redo supports reapplying durable changes, while undo preserves enough prior state to move back toward an earlier logical version.