Free space reallocation optimizes the free space in an aggregate immediately before Data ONTAP writes data to the blocks in that aggregate.
Before Data ONTAP writes data to a segment of blocks in an aggregate, free space reallocation evaluates the layout of those blocks. If the layout is not optimal, the free space reallocation function rearranges the blocks. Rearranging the blocks increases the amount of contiguous free space available in the aggregate, which improves the performance of Data ONTAP writes to those blocks.
The following graphic illustrates how free space reallocation optimizes the free space in a segment of blocks: