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How a reallocation scan works

Data ONTAP performs an aggregate reallocation scan by scanning through an aggregate and reallocating blocks as necessary to improve free-space characteristics.

Data ONTAP performs a file reallocation scan in the following steps:
  1. Scans the current block layout of the LUN.
  2. Determines the level of optimization of the current layout on a scale of 3 (moderately optimal) to 10 (not optimal).
  3. Performs one of the following tasks, depending on the optimization level of the current block layout:
    • If the layout is optimal, the scan stops.
    • If the layout is not optimal, blocks are reallocated sequentially.
    Note: In addition to the specified threshold level, Data ONTAP also includes “hot spots” in its calculation of whether to start a file reallocation. As a result, Data ONTAP might start a reallocation when the average optimization is better than the threshold but a small percentage of the total data is very poorly optimized.
  4. Scans the new block layout.
  5. Repeats steps 2 and 3 until the layout is optimal.

The rate at which the reallocation scan runs (the blocks reallocated per second) depends on CPU and disk loads. For example, if you have a high CPU load, the reallocation scan will run at a slower rate, so as not to impact system performance.