Ubuntu Server 20.04.1 LTS, not all disk space was allocated during installation?
I also used the default Ubuntu 20.04 install from ISO w/ lvm option selected. I had the same problem with the OS disk not occupying what I had allocated. Eddie's suggestion and the provided link did it for me. To summarize:
root@util:~# vgdisplay
<snip>
root@util:~# lvextend -l +100%FREE /dev/mapper/ubuntu--vg-ubuntu--lv
<snip>
root@util:~# resize2fs /dev/mapper/ubuntu--vg-ubuntu--lv
<snip>
Before
root@util:~# df -h
Filesystem Size Used Avail Use% Mounted on
udev 3.9G 0 3.9G 0% /dev
tmpfs 795M 1.1M 793M 1% /run
/dev/mapper/ubuntu--vg-ubuntu--lv 98G 6.5G 86G 8% /
tmpfs 3.9G 0 3.9G 0% /dev/shm
tmpfs 5.0M 0 5.0M 0% /run/lock
tmpfs 3.9G 0 3.9G 0% /sys/fs/cgroup
/dev/xvda2 976M 105M 805M 12% /boot
/dev/loop2 56M 56M 0 100% /snap/core18/1944
/dev/loop0 32M 32M 0 100% /snap/snapd/10707
/dev/loop1 132M 132M 0 100% /snap/docker/796
/dev/loop3 70M 70M 0 100% /snap/lxd/19188
/dev/loop4 33M 33M 0 100% /snap/snapd/11588
/dev/loop5 56M 56M 0 100% /snap/core18/1997
/dev/loop6 71M 71M 0 100% /snap/lxd/19647
tmpfs 795M 0 795M 0% /run/user/1000
After
root@util:~# df -h
Filesystem Size Used Avail Use% Mounted on
udev 3.9G 0 3.9G 0% /dev
tmpfs 795M 1.1M 793M 1% /run
/dev/mapper/ubuntu--vg-ubuntu--lv 196G 6.5G 180G 4% /
tmpfs 3.9G 0 3.9G 0% /dev/shm
tmpfs 5.0M 0 5.0M 0% /run/lock
tmpfs 3.9G 0 3.9G 0% /sys/fs/cgroup
/dev/xvda2 976M 105M 805M 12% /boot
/dev/loop2 56M 56M 0 100% /snap/core18/1944
/dev/loop0 32M 32M 0 100% /snap/snapd/10707
/dev/loop1 132M 132M 0 100% /snap/docker/796
/dev/loop3 70M 70M 0 100% /snap/lxd/19188
/dev/loop4 33M 33M 0 100% /snap/snapd/11588
/dev/loop5 56M 56M 0 100% /snap/core18/1997
/dev/loop6 71M 71M 0 100% /snap/lxd/19647
tmpfs 795M 0 795M 0% /run/user/1000
I had this problem also. The link below has instructions on how to extend the LVM partition to use all available space.
https://www.linuxtechi.com/extend-lvm-partitions/
Not mentioned in the above link is this command to use all remaining space rather than specifying a specific amount to add:
lvextend -l +100%FREE /dev/mapper/ubuntu--vg-ubuntu--lv
The installation is misleading because I specifically selected the option to use all available space, but not all available space was utilized.
Your root filesystem is on an LVM logical volume (LV) /dev/mapper/ubuntu--vg-ubuntu--lv (196 GB).
The LVM volume group (VG) that provides disk space for that LV probably has one single physical volume (PV), presumably that third partition on that SSD (/dev/nvme0n1p3 with 952.4G).
The VG probably still has a lot of disk space that you could use to make that LV bigger or to create another LV. It might be a disk space allocation strategy of the Ubuntu installer not to simply use all available disk space straight away for the root filesystem, giving you some leeway to decide what to do with the remaining disk space.
You didn't use the "-T" argument to the "df" command, so it doesn't tell the filesystem type; but most modern Linux filesystems have a tool to resize them. ext4 certainly supports that.
Here is a tutorial on the LVM tools: https://www.digitalocean.com/community/tutorials/how-to-use-lvm-to-manage-storage-devices-on-ubuntu-18-04
If you didn't explicitly select using LVM during installation, this might also be a result of choosing encryption; that is typically done via LVM.
HTH
Just run this command from the terminal:
sudo resize2fs -p /dev/mapper/ubuntu--vg-ubuntu--lv`
That did it for me. Then check it with:
df -h