Abstract: The present invention provides a method and apparatus for using cache coherency locking to facilitate on-line volume expansion in a multi-controller storage device. A volume set is divided into segments by a first controller. The first controller acquires a cache lock for one of the segments of the volume set to be expanded. A second controller is provided access to any non-cache-locked segments of data during the expansion while safeguarding integrity of the segment undergoing expansion. The first controller also provides the second controller information regarding the expansion so that the second controller may, in the event the first controller fails, perform the expansion using a backup copy of the segment being expanded.