This warns that your thin pool pve/data is currently over-provisioned, i.e. you have assigned more space to volumes that is actually available. As long as there is actual free space nothing is wrong, but once the disks start filling up, it could happen that the pool runs out of space, which is bad.
root@pxmx:~# lvs
LV VG Attr LSize Pool Origin Data% Meta% Move Log Cpy%Sync Convert
cache1 pve Vwi-aotz-- 10.00g data 99.94
cache2 pve Vwi-aotz-- 10.00g data 0.01
data pve twi-aotz-- <141.43g 16.54 1.95
root pve -wi-ao---- 55.75g
snap_vm-100-disk-1_Snap1 pve Vri---tz-k 16.00g data vm-100-disk-1
snap_vm-100-disk-1_Snap2 pve Vri---tz-k 16.00g data vm-100-disk-1
snap_vm-101-disk-0_Snap1 pve Vri---tz-k 20.00g data vm-101-disk-0
snap_vm-101-disk-0_Snap2 pve Vri---tz-k 20.00g data vm-101-disk-0
swap pve -wi-ao---- 7.00g
vm-100-disk-1 pve Vwi-aotz-- 16.00g data 9.71
vm-100-state-Snap1 pve Vwi-a-tz-- <3.49g data 18.07
vm-100-state-Snap2 pve Vwi-a-tz-- <3.49g data 13.01
vm-101-disk-0 pve Vwi-aotz-- 20.00g data 21.14
vm-114-disk-0 pve Vwi-aotz-- 8.00g data 21.16
vm-124-disk-0 pve Vwi-a-tz-- 6.00g data 67.80
vm-124-disk-1 pve Vwi-a-tz-- 4.00g data 1.63
The LSize column shows the provisioned space for each volume and the Data% column shows how much space is currently actually used. The most important thing to keep an eye out is the Data% of the thin pool data itself (currently 16.54).
root@pxmx:~# vgdisplay pve
--- Volume group ---
VG Name pve
System ID
Format lvm2
Metadata Areas 1
Metadata Sequence No 127
VG Access read/write
VG Status resizable
MAX LV 0
Cur LV 16
Open LV 7
Max PV 0
Cur PV 1
Act PV 1
VG Size <223.07 GiB
PE Size 4.00 MiB
Total PE 57105
Alloc PE / Size 53010 / 207.07 GiB <- Space allocated
Free PE / Size 4095 / <16.00 GiB <- Space used
VG UUID ug5DjO-FyBB-O0Gm-rxLW-ibQC-pPrS-jTgo1i
root@pxmx:~# df -h
Filesystem Size Used Avail Use% Mounted on
udev 3.8G 0 3.8G 0% /dev
tmpfs 777M 1.1M 776M 1% /run
/dev/mapper/pve-root 55G 6.3G 46G 13% /
tmpfs 3.8G 46M 3.8G 2% /dev/shm
tmpfs 5.0M 0 5.0M 0% /run/lock
/dev/sda2 511M 328K 511M 1% /boot/efi
zstore 3.5T 128K 3.5T 1% /zstore
zstore/media 3.6T 63G 3.5T 2% /opt/media
/dev/fuse 128M 20K 128M 1% /etc/pve
tmpfs 777M 0 777M 0% /run/user/0