resize an lvm partition on a vm guest

So you've order an upgrade to your virtual disk from your cloud provider and the virtual disk has been extended but df show it is still the same size.

# df -h
Filesystem            Size  Used Avail Use% Mounted on
/dev/mapper/vg_test-lv_root
                       28G  1.4G   26G   6% /
tmpfs                 504M     0  504M   0% /dev/shm
/dev/sda1             477M   48M  404M  11% /boot

e2fsck won't work because it's mounted.

# e2fsck -f /dev/mapper/vg_test-lv_root
e2fsck 1.41.12 (17-May-2010)
/dev/mapper/vg_test-lv_root is mounted.
e2fsck: Cannot continue, aborting.

resize2fs says there is nothing to do!

# resize2fs /dev/mapper/vg_test-lv_root
resize2fs 1.41.12 (17-May-2010)
The filesystem is already 7473152 blocks long.  Nothing to do!

You need to extend the partition with lvextend....

# lvextend -l +100%FREE /dev/mapper/vg_test-lv_root
  Size of logical volume vg_test/lv_root changed from 28.51 GiB (7298 extents) to 78.51 GiB (20098 extents).
  Logical volume lv_root successfully resized

and then resize the file system....

# resize2fs -p /dev/mapper/vg_test-lv_root
resize2fs 1.41.12 (17-May-2010)
Filesystem at /dev/mapper/vg_test-lv_root is mounted on /; on-line resizing required
old desc_blocks = 2, new_desc_blocks = 5
Performing an on-line resize of /dev/mapper/vg_test-lv_root to 20580352 (4k) blocks.
The filesystem on /dev/mapper/vg_test-lv_root is now 20580352 blocks long.

now you can see the space you have ordered!

# df -h
Filesystem            Size  Used Avail Use% Mounted on
/dev/mapper/vg_test-lv_root
                       78G  1.4G   72G   2% /
tmpfs                 504M     0  504M   0% /dev/shm
/dev/sda1             477M   48M  404M  11% /boot

As always such operations carry a risk so take a backup or better still a snapshot and carry out at your own risk.