a method is provided for operating on a transaction object in a controller , such as a disk array controller . the transaction object maintains a roll forward log which holds action items to be performed , and a roll back log which lists action items already performed . in the event of power failure , the controller might be in the midst of a complex operation involving many action items . when power is restored , the controller determines whether to complete the operation using the roll forward log , or undo the operation using the roll back log . the method includes a simplified sequence , keyed to a single instruction of clearing the roll back log , which effectively makes this determination and thereby renders atomic the entire operation with respect to power failure .