What is preservation capacity?
Preservation capacity is the amount of capacity (number of drives) that is reserved in a pool to support potential drive failures.
Why would I logically replace a drive? If a drive fails or you want to replace it for any other reason, and you have an unassigned drive in your storage array, you can logically replace the failed drive with the unassigned drive. If you do not have an unassigned drive, you can physically replace the drive instead.