Used vgextend/lvextend to add addtional 8GB space but it is not reflected in df output

Solution 1:

Output is absolute in the line

sudo lvextend -l 100%FREE /dev/ubuntudevbox2/root

Since you only had 8G of free extents after you vgextended you only get a lv representing that absolute size of 8G.

You must inform LVM you intend to make an additional space size. From the manpage of lvextend;

   -l, --extents [+]LogicalExtentsNumber[%{VG|LV|PVS|FREE|ORIGIN}]
          Extend  or  set  the  logical  volume  size  in units of logical
          extents.  With the '+' sign the value is  added  to  the  actual
          size of the logical volume and without it, the value is taken as
          an absolute one.

Try

sudo lvextend -l +100%FREE /dev/ubuntudevbox2/root

Solution 2:

As you can see from your vgdisplay output only about a half of space is allocated:

Alloc PE / Size       2182 / 8.52 GiB
Free  PE / Size       1851 / 7.23 GiB

I suspect you need to specify parameters for lvextend more explicitly.