Sun ZFS Storage 7420 inconsistent size nfs filesystem

We have Sun ZFS Storage 7420 sharing some nfs filesyste with Exdata X3 machine. These filesystem don't display whole avalaible space when we executed du -h command.  Moreover we had errors in RMAN backups process : "no space left on device". For example :

snapshot yesterday:

Filesystem

Size  Used Avail Use% Mounted on

1.100.100.100:/export/rman_backups/backup101    36T  5.3T   31T  15% /usr/local/pp/exazfs/backup101

snapshot today:

Filesystem

Size  Used Avail Use% Mounted on

1.100.100.100:/export/rman_backups/backup101    12T   11T  824G  93% /usr/local/pp/exazfs/backup101

As we can see, we "lost" 24T of disk space.

Does anybody know what it is happing? any idea?