Storage increase its own capacity

[Buso]

Dears,
My FreeNAS installation (FreeNAS-11.2-U8 ) has one storage attached located on a SAN where the data are recorded.
Sometimes this storage is full and FreeNAS does not work: data recorded in this storage is 3% of the full size but for an unknown reason, the data occupied increases.

Do you know why the size of the storage attached is increasing without any clear reason?

Thanks