Background
1. The partition is ordinary partition and not LVM
2. The filesystem is ext4
3. The steps tested on proxmox 5 and 6 and maybe applicable to other versions of Proxmox
Step by step
1. Select and resize the disk you want in proxmox gui
2. Login to the VM and type dmesg command to see the relevant device that has changed i.e /dev/vde
3. unmount the partition i.e /dev/vde1 : umount /dev/vde1
4. use parted to extend the disk's size:
4.1 parted /dev/vde
4.2 resizepart 1
4.3 q
5. unmount the partition if it is automatically mounted. If it is not, then proceed to the next step:
umount /dev/vde1
6. run filesystem check: e2fsck -f /dev/vde1
7. filesystem resize: resize2fs /dev/vde1
8. mount the partition i.e mount to /data: mount /dev/vde1 /data
9. Check and verify
That's all there is to it. Have fun!
