35
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