Proxmox: Shrinking disk of an LVM backed container

LVM, or Logical Volume Management, is a storage device management technology that gives users the power to pool and abstract the physical layout of component storage devices for easier and flexible administration. Utilizing the device mapper Linux kernel framework, the current iteration, LVM2, can be used to gather existing storage devices into groups and allocate logical units from the combined space as needed.

First, shut down the container and ensure it’s not running.

pct list
VMID       Status     Lock         Name                
100        stopped                 100.pve.prado.lt 

List out the LVM logical volumes:

lvdisplay | grep "LV Path\|LV Size"
  LV Path                /dev/pve/swap
  LV Size                3.62 GiB
  LV Path                /dev/pve/root
  LV Size                7.25 GiB
  LV Size                13.00 GiB
  LV Path                /dev/pve/vm-100-disk-0
  LV Size                20.00 GiB

Choose the disk you want to resize – you’ll see they’re named by container ID and disk number. For example, /dev/pve/vm-100-disk-0.

Check and fix the file system, just in case:

e2fsck -fy /dev/pve/vm-100-disk-0
e2fsck 1.43.4 (31-Jan-2017)
Pass 1: Checking inodes, blocks, and sizes
Pass 2: Checking directory structure
Pass 3: Checking directory connectivity
Pass 4: Checking reference counts
Pass 5: Checking group summary information
/dev/pve/vm-100-disk-0: 24382/1310720 files (0.0% non-contiguous), 268496/5242880 blocks

Resize the file system. It is advisable, at this point, to set this to 10 GB smaller than you actually want it (e.g. 9G when you want 10G):

resize2fs /dev/pve/vm-100-disk-0 9G
2fsck 1.43.4 (31-Jan-2017)
Pass 1: Checking inodes, blocks, and sizes
Pass 2: Checking directory structure
Pass 3: Checking directory connectivity
Pass 4: Checking reference counts
Pass 5: Checking group summary information
/dev/pve/vm-100-disk-0: 24382/1310720 files (0.0% non-contiguous), 268496/5242880 blocks
[email protected]:~# resize2fs /dev/pve/vm-100-disk-0 9G
resize2fs 1.43.4 (31-Jan-2017)
Resizing the filesystem on /dev/pve/vm-100-disk-0 to 2359296 (4k) blocks.
The filesystem on /dev/pve/vm-100-disk-0 is now 2359296 (4k) blocks long.

Resize the LVM LV to the actual size you want it to be:

lvreduce -L 10G /dev/pve/vm-100-disk-0

Resize the file system to fill the LVM LV:

resize2fs /dev/pve/vm-100-disk-0

Finally, edit the configuration for the container such that Proxmox reports the correct size for the disk. You will find this at /etc/pve/lxc/100.conf where 100 is your container ID.
Before changes:

arch: amd64
cores: 1
hostname: 100.pve.prado.lt
memory: 512
net0: name=eth0,bridge=vmbr0,firewall=1,hwaddr=AE:0B:58:33:E6:F4,ip=dhcp,ip6=dhcp,type=veth
ostype: debian
rootfs: local-lvm:vm-100-disk-0,size=20G
swap: 512
unprivileged: 1

After changes:

arch: amd64
cores: 1
hostname: 100.pve.prado.lt
memory: 512
net0: name=eth0,bridge=vmbr0,firewall=1,hwaddr=AE:0B:58:33:E6:F4,ip=dhcp,ip6=dhcp,type=veth
ostype: debian
rootfs: local-lvm:vm-100-disk-0,size=10G
swap: 512
unprivileged: 1

You can now start your container and check the disks’ sizes:

pct start 100
pct enter 100
df -h
Filesystem                        Size  Used Avail Use% Mounted on
/dev/mapper/pve-vm--100--disk--0  9.8G  592M  8.7G   7% /
none                              492K     0  492K   0% /dev
udev                              1.9G     0  1.9G   0% /dev/tty
tmpfs                             2.0G     0  2.0G   0% /dev/shm
tmpfs                             2.0G  8.1M  2.0G   1% /run
tmpfs                             5.0M     0  5.0M   0% /run/lock
tmpfs                             2.0G     0  2.0G   0% /sys/fs/cgroup

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