Techniques are provided for guaranteed undo retention. The techniques include a database server attempting to store undo information in undo storage. In order to store the undo information, the database server may overwrite expired undo records; write to empty undo extents; allocate new undo extents and write to the newly-allocated extents; or overwrite unexpired undo records that do not have guaranteed undo retention. If an undo record does not have guaranteed undo retention, then the undo record may be overwritten before the expiration time has elapsed. If no undo extent is empty or unallocated, and if all of the undo records are unexpired and have guaranteed undo retention, then the database system has, at least temporarily, run out of usable and reusable space and an error may be reported to an appropriate party.