Question: I just installed the new one Debian xfce via VirtualBox, but decided to increase the vdi size from 8GB to 20GB (because it requires 42% of all disk space at this moment):

> 
> VBoxManage.exe modifyhd debian.vdi --resize 20000
> 
> 
> 

VirtualBox Manager tells me that debian.vdi now has 20GB as supposed, but when I started Debian, the df output tells me that it has 8GB as earlier. Why?

Answer: 
I think you need to resize the filesystem with resize2fs
For example:

```
resize2fs /dev/sda1

```

/edit:

@Stephane Chazelas is correct. You need to resize the partition first. Then resize the filesystem. If you use lvm it's failry simple. Create a new partition with fdisk on the new empty space, use lvm type (8e). Then create a physical volume on that new partition with 

`pvcreate <path_to_new_disk>`

Then add the new physical volume to the volume group (use `vgs` to see a summary of volume groups):

`vgextend <volumegroup_name> /path_to_new_disk`

Then you can extend the lv (run `lvs` to see a list of logical volumes):
`lvextend <path_to_current_logical_volume> <path_to_new_physical_volume>`

For example. If the current disk that you whant to resize is /dev/sda1 and the new space is on /dev/sda2 (where volgroup is the volume group name):

```
pvcreate /dev/sda2
vgextend volgroup /dev/sda2
lvextend /dev/volume_group_name/logical_volume_name /dev/sda2

```

Now the partition is resized, the next step is to resize the filesystem with resize2fs:

`resize2fs /dev/volume_group_name/logical_volume_name>`
You have to distinguish the disk space and the size of the partition. 

If you resize a `vdi` it means you enlarge your physical disk but in the virtual OS Debian, as long as you do not resize the logical partition then your OS will not be able to write on the new physical space available. 

To accomplish such a task, you can use the binary `resize2fs` [see the man to identify your needs](http://linux.die.net/man/8/resize2fs) or if you don't feel comfortable with command line you can download `gparted` bootable iso and use `gparted` (which is graphical) by booting on the `iso` to resize easily your Debian partition. 

The question has already been answer several time : 

* Ask Ubuntu "[How do I repartition with GParted? [duplicate]](https://askubuntu.com/q/51272/93232)" which was a duplicate from "[How to resize partitions?](https://askubuntu.com/questions/126153/how-to-resize-partitions)"
* see also this post from U&L Q&A titled: "[Resize partitions with gparted](https://unix.stackexchange.com/a/83313/53092)"