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Reasons to use physical reallocation scans

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.

Note: Physical reallocation is not supported on flexible volumes or on files and LUNs within flexible volumes that are in an aggregate created by a version of Data ONTAP earlier than version 7.2.
Note: Using the -p option might cause a performance degradation when reading older Snapshot copies, if the volume has significantly changed after reallocation. Performance might be impacted when reading files in the .snapshot directory, accessing a LUN backed up by a Snapshot copy, or reading a qtree SnapMirror (QSM) destination. This performance degradation does not occur with whole-volume reallocation.