Reducing the size of /dev/pve/root (the root logical volume in a default Proxmox setup using LVM) and extend the space of /dev/pve/data can be done using SystemRescueCD (or any live Linux environment with LVM and filesystem tools).

This is risky and should only be done after a full backup of your Proxmox host and VMs.

Warnings

  • Shrinking a mounted filesystem is dangerous — always do it from a live system (eg: SystemRescueCD).

  • Back up everything before proceeding.

  • These steps assume you’re using ext4.

Boot into SystemRescueCD

Check current LVM layout

lvdisplay

  --- Logical volume ---
  LV Path                /dev/pve/root
  LV Name                root
  VG Name                pve
  LV Write Access        read/write
  LV Status              available
  # open                 1
  LV Size                64.00 GiB
  Current LE             8192
  Segments               1
  Allocation             inherit
  Read ahead sectors     auto
  - currently set to     256
  Block device           253:0

  --- Logical volume ---
  LV Path                /dev/pve/data
  LV Name                data
  VG Name                pve
  LV Write Access        read/write
  LV Status              available
  # open                 1
  LV Size                10.88 TiB
  Current LE             2852702
  Segments               1
  Allocation             inherit
  Read ahead sectors     auto
  - currently set to     256
  Block device           253:1

Activate LVM volumes

lvscan
vgchange -ay

Check the filesystem

Assuming /dev/pve/root is ext4

e2fsck -f /dev/pve/root

Shrink the filesystem of /dev/pve/root

Decide how much you want to reduce it by.

Example: shrink /dev/pve/root to 32G.

lvreduce --resizefs --size 32G /dev/pve/root

Expand the filesystem of /dev/pve/data to use (now) available free space

lvresize --resizefs --extents +100%FREE /dev/pve/data

Run e2fsck again (optional but recommended)

e2fsck -f /dev/pve/root
e2fsck -f /dev/pve/data

Reboot

Remove or unmount SystemRescueCD and (re)boot from internal drive.

Check syslog for any relevant errors.

Tip: If you need assistance or support for a similar issue, you can contact us ;)