In a method for allocating space on a logical disk, a computer receives an allocation request to allocate a number of requested logical disk extents. The computer selects one of a first group having an array of logical disk extents and a second group having an array of logical disk extents. The computer selects a group having a number of free logical disk extents that is greater than or equal to the number of requested logical disk extents. The logical disk extents in the array of the first group and in the array of the second group correspond to disk blocks on a logical disk. The logical disk spans one or more physical random access disks. The computer locks the selected group to prevent allocating a logical disk extent other than in response to the allocation request.