Ceph » CephFS

hi~, I ask for some question about "df -h" after "ceph osd pool set-quota"

CentOS Linux release 8.4.2105
ceph version 16.2.5 (0883bdea7337b95e4b611c768c0279868462204a) pacific (stable)

1、I create a pool named "pool_a", there were some other pools in cluster, total size was 100G
2、I executed "ceph osd pool set-quota" to limited the pool_a's size to 20G
3、I executed "ceph osd pool get-quota" to show the pool_a's quota, yes, it worked
4、I created a fs named "cephfs" by the pool "pool_a"
5、I mount the fs to /mnt
6、I executed "df -hT", it showed that the /mnt max size was not the quote but the size of the cluster

I expected the limit size of fs was equal to the pool's quota, so need I execute other operations to achieve my goal？