A reallocation scan evaluates how the blocks are laid out on disk in a LUN, file, volume, or aggregate, and rearranges them if necessary.
Data ONTAP performs the scan as a background task, so applications can rewrite blocks in the LUN, file, volume, or aggregate during the scan. Repeated layout checks during a file, LUN, or volume reallocation scan ensure that the sequential block layout is maintained during the current scan.
A reallocation scan does not necessarily rewrite every block in the LUN, file, or volume. Rather, it rewrites whatever is required to optimize the block layout.
The more free space the target has, the more effective the reallocation scan is.
For information about volume guarantees, see the Data ONTAP Storage Management Guide for 7-Mode.
When you use reallocate start without the -p option, a reallocation scan duplicates blocks that are held in a Snapshot copy, so a file might use more space after the scan. When you use reallocate start with the -p option, blocks are moved, and the file takes up less additional space after the scan.