Expand LVM ext4 root filesystem

On a basic Ubuntu Server install, an Oops moment:

# df -h /
Filesystem                                   Size  Used Avail Use% Mounted on
/dev/mapper/ubuntu--vg-ubuntu--lv            7.9G  7.5G   21M 100% /

Fortunately there is free space in the volume group:

# vgs
  VG        #PV #LV #SN Attr   VSize  VFree
  ubuntu-vg   1   7   0 wz--n- <1.72t <62.81g

Extend the logical volume by 10G

# lvextend -L +10G /dev/mapper/ubuntu--vg-ubuntu--lv
  Size of logical volume ubuntu-vg/ubuntu-lv changed from 8.00 GiB (2048 extents) to 18.00 GiB (4608 extents).
  Logical volume ubuntu-vg/ubuntu-lv successfully resized.

Update - LVM needs a small amount of free space to expand a LV. If there really is no free space then you might see this error:

# lvextend -L +10G /dev/ubuntu-vg/ubuntu-lv
  /etc/lvm/archive/.lvm_srv05_21685_822342773: write error failed: No space left on device

In this case the simplest approach is to delete a few files to free up a very small amount of space,
e.g. remove old syslog files.

# rm /var/log/syslog.*gz

Resize the LVM path - on XFS we would grow the filesystem mount point.

# resize2fs /dev/ubuntu-vg/ubuntu-lv
resize2fs 1.44.6 (5-Mar-2019)
Filesystem at /dev/ubuntu-vg/ubuntu-lv is mounted on /; on-line resizing required
old_desc_blocks = 1, new_desc_blocks = 3
The filesystem on /dev/ubuntu-vg/ubuntu-lv is now 4718592 (4k) blocks long.

Check for errors.

# dmesg | grep EXT4
[12312064.327744] EXT4-fs (dm-0): resizing filesystem from 2097152 to 4718592 blocks
[12312064.755646] EXT4-fs (dm-0): resized filesystem to 4718592

Check filesystem size.

# df -h /
Filesystem                         Size  Used Avail Use% Mounted on
/dev/mapper/ubuntu--vg-ubuntu--lv   18G  7.4G  9.6G  44% /