Recovery: The UNDO Phase

ToUndo={ l | l a lastLSN of a “loser” Xact}

Repeat:

	Choose largest LSN among ToUndo.
	If this LSN is a CLR and undonextLSN==NULL Write an End record for this Xact.
	If this LSN is a CLR, and undonextLSN != NULL Add undonextLSN to ToUndo (Q: what happens to other CLRs?)
	Else this LSN is an update. Undo the update, write a CLR, add prevLSN to ToUndo.