By default when snapshot is created on volume with LUN Data ONTAP reserves amount of space that is “frozen” in snapshot on volume to ensure LUN still has space to write into it. Please read TR-3483 which explains all this in great details.
- Snap reserve is meaningless on volumes containing LUN and simply reduce useable space
- Automatic (filer initiated) snapshots of volumes with LUNs will often result in inconsistent data on LUNs; the effect varies from hard to recover up to impossible to recover data from snapshot. Usually snapshots are initiated from host in coordination with applications on LUNs. You have automatic snapshots turned on.