Described is a technology by which a new volume or partition may be created on a disk, e.g., by running a shrink program and then reclaiming freed space. Shrink occurs online, while the user or system processes may be otherwise using the disk for reads and writes. Further, the technology operates while protecting snapshot versions of the volume. To shrink, upon receiving a request to shrink a volume to within a boundary, new allocations are restricted such that any allocation is to a volume area within the boundary. Data is moved from outside the boundary to within the boundary, and the shrink is committed when no volume data remains outside the boundary. A reduced-size volume or partition that does not include the specified region is committed when the data has been moved out of the specified region. A new volume or partition may be freed that corresponds to the region.