Is there a way to clean large aggregate footprint

Hello,

We encounter a strange issue on 8.2.4 7 mode.

An aggregate is filled with more than 3 TB of datas,

Aggregate               kbytes       used      avail capacity  
aggr0               21055590144 3609363136 17446227008      17%  
aggr0/.snapshot              0          0          0       0%

and however there are only 2 volumes in that aggregate of around 250 Go:

Filesystem              kbytes       used      avail capacity  Mounted on
/vol/volboot/        249036800    6923172  242113628       3%  /vol/volboot/
/vol/volboot/.snapshot   13107200     112104   12995096       1%  /vol/volboot/.snapshot
/vol/volpraboot/     249036800    4945480  244091320       2%  /vol/volpraboot/
/vol/volpraboot/.snapshot   13107200      31516   13075684       0%  /vol/volpraboot/.snapshot

 We can see that this amount of space is consumed of footprint:

Aggregate : aggr0

      Feature                                           Used      Used%
      --------------------------------      ----------------      -----
      Volume Footprints                               3.36TB        17%
      Aggregate Metadata                              1.85MB         0%

      Total Used                                      3.36TB        17%

For information, this aggregate was build with disks from a new shelf, then vol boot was moved into that new shelf (ndmpcopy), filer restarted, and then original aggr0 was deleted.

Should the volume footprints of aggr0 be the sum of each volume footprint ? It doesn't seem to be the case as individual volume foot print do not represent 3 TB

Is there a way to reduce the size of these footprint ?

Many thanks for your help,

Regards