You can optimize the existing layout of a LUN, a file, a volume, or an aggregate.
Optimizing the existing layout of a LUN, file, or volume improves the sequential read performance of host applications that access data on the storage system. Write performance may also be improved as a result of file reallocation. Optimizing the layout of a volume is equivalent to optimizing all files and LUNs in the volume.
Optimizing the existing layout of an aggregate improves contiguous free space in the aggregate, hence improving the layout, and usually the performance, of future writes to volumes in the aggregate. Optimizing the aggregate layout is not equivalent to optimizing all the volumes in the aggregate.