Why did I allocate 32G hard disk to the virtual machine, but Ubuntu only recognizes 16G?

Preface

Recently discovered my Ubuntu server freezing inexplicably

Logged in to check and found the hard drive was full

But using df -h revealed something strange—I had clearly allocated 32G of space in PVE, yet Ubuntu only showed 16G

$ df -h
/dev/mapper/ubuntu--vg-ubuntu--lv   15G   14G     0  100% /

Wait, what? I allocated 32G, so why is only 15G showing? Didn't half the space just vanish into thin air?

Initially thought it was a disk configuration error in PVE, but later realized—it really wasn't PVE's fault.

Very strange.

After some research, the reason turned out to be that the virtual disk allocated by PVE uses LVM.

The entire situation involves the layered relationship between virtualization disk, partition, LVM logical volume, and file system.

The Truth

Originally, Ubuntu enables LVM (Logical Volume Manager) by default during installation.

The logic of LVM is as follows: First, the entire disk is configured as a "Volume Group" (VG), and then several "Logical Volumes" (LVs) are created from it to be mounted and used.

The problem arises here—the installer, by default, allocates only about half of the space to the root directory.

Thus, although the virtual disk is 32GB, the root directory only received 16GB, while the remaining space quietly sits unused in the Volume Group.

It's somewhat like buying a 32GB USB drive but only creating a single 16GB partition to use, leaving the rest forgotten in a corner.

Expansion

Now that you know the cause, it's easy to fix. Just three simple steps:

Execute on the Ubuntu server:

# 1. Check the remaining space in the volume group
# Here it displays Free PE / Size, which is the unallocated space in the volume group (LVM).
vgdisplay
# 2. Expand the root logical volume to use all remaining space
# Here, +100%FREE means to allocate all remaining space in the volume group to this logical volume.
lvextend -l +100%FREE /dev/ubuntu-vg/ubuntu-lv
# 3. Extend the Filesystem to Actually Utilize the Space
resize2fs /dev/ubuntu-vg/ubuntu-lv

Here, the default filesystem is ext4, so we use the resize2fs command.

For xfs, use xfs_growfs /.

Then let's check with df -h... Perfect!

/dev/mapper/ubuntu--vg-ubuntu--lv   31G   14G   17G  45% /

Summary

This time, I got a lesson from LVM

I hadn't really used LVM before

Next time you see a situation like "allocated 32G but only showing 15G," you can try the solution described in this article.