I have changed the vdi size from 20 GB to 40 GB, but the increase has no effect on my ubuntu machine, it still says the memory is as it was earlier. Two screen shots have been provided, the host is windows 7 professional, the guest is ubuntu 14.04 LTS, the virtual machine is oracle vm virtualbox.
the screen shot is of storage of virtual machine settings
the output of df -H command in ubuntu terminal
The question is how I can have the effect of resizing vdi on ubuntu machine.
The problem here is that you've increased the disk size within VirtualBox, but not within the Ubuntu operating system within the guest environment. You can achieve this by having the guest boot into some disk management software, I would suggest using GParted.
So you'd get the gparted.iso file on your host system, attach it to your vm as an optical disk (dvd), and boot the VM into it. Using Gparted, you would expand the partition which contains your root filesystem. When you detach gparted and boot the VM back as Ubuntu, you will see the new space being used.
Gparted is pretty safe, but it's always worth making a backup before doing anything with disks and partitions, even if they're virtual!
