lun space bigger than parent volume

Hi guys,


I'm a little bit confused about how ontap manage space. I have been testing man things and a strange thing ocurred. Here it is:


- Create a volume: 50 GB (vol_A)

- Create a LUN: 10 GB (lun_1 --> Parent volume is 'vol_A') using SnapDrive and add to Cluster Shared Volumes.


For now everything is okay. I can see in OnCommand System Manager that the volume available space is 40 GB.


The problem comes when I copy some file in that LUN (5 GB file for example). When I go back to OnCommand System Manager I see that the volume available space is 42, 43 or 44 GB (it's not exactly everytime). Then If I expand the LUN using SnapDrive the maximum size of the LUN that I can expand is 54 GB ----- (10 GB LUN + volume available space)...


Could you help me?


It's annoying because I don't have the control of the real space.


Thank you very much,