If you're wanting the amount of capacity consumed in the volume to reflect the amount of space in the LUN you'll need to use thin provisioning for the LUN. However, this will only get you part of the way...by default LUNs don't shrink, i.e. if you write 1TB of data to a 2TB LUN, then delete that data the LUN will still consume 1TB of capacity in the FlexVol. To reclaim that space you will need to UNMAP from the guest OS. See this KB article for more information.
Andrew
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