No free space on ubuntu 16.04 running on virtual box

I'm having hard times for a couple of weeks with this one... I'm using Ubuntu 16.04.2 LTS as a dev web server, it's running on virtualbox (win7) and these "no free space" available are coming again and again. I tried to remove every temp files, got it working for a few hours, then the same issue again (no surprise here). I googled it and tried to extend my disk size. I managed to extend it, allocate the new free space to my partition with gparted live cd (I'm running ubuntu in terminal mode, no startx installed) I thought I walked through this, it worked well for a few days, then no free space again... I doubled the disk size, so I'm kind of lost here...

IMO, my disk should have enough free space... I must have missed something obvious...

df -h gives me :

Filesystem Size Used Avail Use% Mounted on
udev 981M 0 981M 0% /dev
tmpfs 201M 3.3M 197M 2% /run
/dev/mapper/ubuntu--vg-root 5.4G 5.0G 23M 100% /
tmpfs 1001M 0 1001M 0% /dev/shm
tmpfs 5.0M 0 5.0M 0% /run/lock
tmpfs 1001M 0 1001M 0% /sys/fs/cgroup
/dev/sda1 472M 105M 343M 24% /boot
tmpfs 201M 0 201M 0% /run/user/1000

EDIT:
sudo du -ks /*

15940 /bin
104844 /boot
0 /dev
7072 /etc
895620 /home
0 /initrd.img
0 /initrd.img.old
636860 /lib
4 /lib64
16 /lost+found
8 /media
4 /mnt
4 /opt
du: cannot access '/proc/2921/task/2921/fd/4': No such file or directory
du: cannot access '/proc/2921/task/2921/fdinfo/4': No such file or directory
du: cannot access '/proc/2921/fd/4': No such file or directory
du: cannot access '/proc/2921/fdinfo/4': No such file or directory
0 /proc
16 /root
3332 /run
13232 /sbin
4 /snap
4 /srv
0 /sys
32 /tmp
1484508 /usr
2181768 /var
0 /vmlinuz
0 /vmlinuz.old

lsblk

NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT
sda 8:0 0 14.7G 0 disk
├─sda1 8:1 0 487M 0 part /boot
├─sda2 8:2 0 1K 0 part
└─sda5 8:5 0 14.2G 0 part ├─ubuntu--vg-root 252:0 0 5.5G 0 lvm / └─ubuntu--vg-swap_1 252:1 0 2G 0 lvm [SWAP]
sr0 11:0 1 1024M 0 rom

2 Answers

Thanks to @user4556274, I read the answer he gave here, I just used the command :

sudo lvm lvextend -r -l +100%FREE /dev/ubuntu-vg/root

and I got the free space I needed

If you are running Ubuntu, you may consider to user "Disk Usage Analyzer" app which comes bundled with it. This gives better picture of usages and helps locate folders directly.

I was facing same issue and found that /var/tmp was almost using 90G of temporary files with the help of Disk Usage Analyzer.