How to optimize LUN, file, volume, and aggregate layout

You can optimize the existing layout of a LUN, a file, a volume, or an aggregate.

Optimizing the existing layout of a LUN, file, or volume improves the sequential read performance of host applications that access data on the storage system. Write performance may also be improved as a result of file reallocation. Optimizing the layout of a volume is equivalent to optimizing all files and LUNs in the volume.

Optimizing the existing layout of an aggregate improves contiguous free space in the aggregate, hence improving the layout, and usually the performance, of future writes to volumes in the aggregate. Optimizing the aggregate layout is not equivalent to optimizing all the volumes in the aggregate.

• What a reallocation scan is
  A reallocation scan evaluates how the blocks are laid out on disk in a LUN, file, volume, or aggregate, and rearranges them if necessary.
• Reasons to use LUN, file, or volume reallocation scans
  You run LUN, file, or volume reallocation scans to ensure that blocks in a LUN, file, or volume are laid out sequentially.
• Reasons to use aggregate reallocation scans
  You run aggregate reallocation scans to optimize the location of physical blocks in the aggregate. Doing so increases contiguous free space in the aggregate.
• 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.
• 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.
• How you manage reallocation scans
  To manage reallocation scans, you must enable reallocation scans on your storage system. Then you define a reallocation scan to run at specified intervals or on a specified schedule.
• How to use reallocation scans most efficiently
  To maximize efficiency, you should follow certain guidelines when using reallocation scans.