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For an aggregate to reclaim the entire allocated space back from the volume (which you don't want to have anymore and the data it contains), you have to destroy the volume after taking it offline. Just taking the volume offline will not fetch you the entire vol space back. When you take a FlexVol volume offline, it gives back any unused space that has been allocated for it in its containing aggregate. If this space is allocated for another volume and then you bring the volume back online, this can result in an overcommitted aggregate.