Cannot allocate 2TB disk space while over 3TB is free

I'm trying to add a large disk to a vm, There's plenty of space on the zfs pool (tank2) according to df or the Proxmox UI. Still when I try to add a disk greater than 1TB I get an error:

zfs error: cannot create 'tank2/vm-111-disk-2': out of space at /usr/share/perl5/PVE/API2/Qemu.pm line 1442. (500)

df reports 3.8T available, while the UI says it's got 4.71 TB (in the add file dialog).

Still, adding 2TB fails...

Why is this and why do df and the UI report different values?

Thanks...

Read more