Why does not see the new size after increasing the disk?

Question

linux

Good afternoon, here are some misunderstandings:
If you increase the size of the second disk C:\Program Files\Oracle\VirtualBox
VBoxManage.exe modifyhd C:\Users\root\VirtualBoxVMs\nginx\2.vdi --resize 20480
then after rebooting the OS sees a new the size

sdb                           8:16   0   20G  0 disk 
├─sdb1                        8:17   0   10G  0 part 
│ └─VolGroup-lv_root (dm-0) 253:0    0 16.5G  0 lvm  /
└─sdb2                        8:18   0   10G  0 part

but there is a problem when I increase the main disk.
C:\Program Files\Oracle\VirtualBox
VBoxManage.exe modifyhd C:\Users\root\VirtualBoxVMs\nginx\1.vdi --resize 20480
done...
As you can see, the disk has grown, but the OS does not see how it was above.

sda                              8:0    0    8G  0 disk 
├─sda1                        8:1    0  500M  0 part /boot
└─sda2                        8:2    0  7.5G  0 part 
  ├─VolGroup-lv_root (dm-0) 253:0    0 16.5G  0 lvm  /
  └─VolGroup-lv_swap (dm-1) 253:1    0  992M  0 lvm  [SWAP]

how to make it see that there is free space?

Answer 1

You have expanded the disk, but the logical partition (sda2) has not.

Answer 2

From the root, we give the command: resize2fs /dev/VolGroup-lv_root (the partition on which you have mounted / ) and wait for the end of the process. Takes some time.