Proxmox Ubuntu Disk Resize
If you have installed Ubuntu on Proxmox there is a chance at somepoint that your /dev/mapper partition will become full.
To expand it. Log into Proxmox, Resize the LVM by adding xG.
SSH onto your Ubuntu VM
df -h
(shows the issue)
/dev/mapper/ubuntu--vg-ubuntu--lv 19G 13G 4.8G 74% /
fdisk -l
sudo /sbin/lvresize -l +100%FREE /dev/mapper/ubuntu--vg-ubuntu--lv
sudo /sbin/resize2fs /dev/mapper/ubuntu--vg-ubuntu--lv
Done