The Proxmox Web-UI is great but it doesn’t allow for reducing the size of a VM disk or Container disk that you may have overdone. Here’s how to do it.

You will need access to your Proxmox node via SSH or directly.

This applies to the standard Proxmox setup using LVM.

On your Proxmox node, do the following:

List containers

pct list

Stop the container you want to resize

pct stop 420

Find out it’s path on the node

lvdisplay | grep "LV Path\|LV Size"

Run a file system check

e2fsck -fy /dev/pve/vm-420-disk-0

Resize the file system

resize2fs /dev/pve/vm-420-disk-0 10G

Resize the local volume

lvreduce -L 10G /dev/pve/vm-420-disk-0

Edit the container’s conf file

nano /etc/pve/lxc/420.conf

Update the following line accordingly

FROM:

rootfs: local-lvm:vm-420-disk-0,size=20G

TO:

rootfs: local-lvm:vm-420-disk-0,size=10G

Start the container

pct start 420

Enter and check the resize container disk

pct enter 420
df -h

Source: Proxmox Forum [Online] Available: https://forum.proxmox.com/threads/resize-lxc-disk-on-proxmox.68901/