A physical reallocation (using the -p option of the reallocate start command) reallocates user data on the physical blocks in the aggregate, while preserving the logical block locations within a flexible volume. You can perform physical reallocation with flexible volumes or files and LUNs within flexible volumes.
Physical reallocation might reduce the extra storage requirements in a flexible volume when reallocation is run on a volume with Snapshot copies. It might also reduce the amount of data that needs to be transmitted by SnapMirror on its next update after reallocation is performed on a SnapMirror source volume.