zfs filesystem ( used size didnt reduce after moving oracle datafile )

Hi Expert, 

I have a case when moving a big oracle datafile 10G from source filesysem to other filesystem, 

after succesfully moved, the utilization of source filesystem still not reduced  

#ls -l /oracle/AOG/sapdata2/sr3_5/sr3.data5

-rw-r-----  1 oraaog  dba   10485768192 Jun 15 09:06 /oracle/AOG/sapdata7/sr3_5/sr3.data5

before moving datafile to /oracle/SID/sapdata7

# df -h /oracle/SID/sapdata2

Filesystem       Size  Used Available Capacity Mounted on

poolAOG/sapdata2    49G  30G    19G  61%  /oracle/AOG/sapdata2

after moving datafile to /oracle/SID/sapdata7,the used size still 30G 

# df -h /oracle/SID/sapdata2

Filesystem       Size  Used Available Capacity Mounted on

poolAOG/sapdata2    49G  30G    19G  61%  /oracle/AOG/sapdata2