Gfilui

My question differs from The volume "Filesystem Root" has only 533.9 MB disk space remaining, for mine is specific about managing disk space on multiboot devices

From time to time, the system shows this warning message:

I have a dual-boot with Windows 10 and Ubuntu 16.04. Here's what fdisk shows:

mike@mike-thinks:~$ sudo fdisk -l
[sudo] password for mike: 
Disk /dev/sda: 931,5 GiB, 1000204886016 bytes, 1953525168 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Disklabel type: dos
Disk identifier: 0xc3c0f778

Device Boot Start End Sectors Size Id Type
/dev/sda1 * 2048 1026047 1024000 500M 7 HPFS/NTFS/exFAT
/dev/sda2 1026048 1743806463 1742780416 831G 7 HPFS/NTFS/exFAT
/dev/sda3 1743806464 1782867967 39061504 18,6G 83 Linux
/dev/sda4 1782870014 1953523711 170653698 81,4G 5 Extended
/dev/sda5 1782870016 1798492159 15622144 7,5G 82 Linux swap / Solaris
/dev/sda6 1798494208 1953523711 155029504 73,9G 83 Linux

Partition 4 does not start on physical sector boundary.

Disk /dev/mapper/cryptswap1: 7,5 GiB, 7998013440 bytes, 15621120 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
mike@mike-thinks:~$ sudo df -h
Filesystem Size Used Avail Use% Mounted on
udev 3,9G 0 3,9G 0% /dev
tmpfs 789M 20M 770M 3% /run
/dev/sda3 19G 17G 418M 98% /
tmpfs 3,9G 58M 3,8G 2% /dev/shm
tmpfs 5,0M 4,0K 5,0M 1% /run/lock
tmpfs 3,9G 0 3,9G 0% /sys/fs/cgroup
/dev/sda6 73G 63G 6,2G 92% /home
tmpfs 789M 60K 789M 1% /run/user/1000
/home/mike/.Private 73G 63G 6,2G 92% /home/mike

What would you advise me to do ?

Yes, you should definitely increase the size of root from 39GB. Suggest you

A) Make two backups of each partition with data

B) Do

sudo apt-get autoclean && sudo apt-get autoremove && sudo apt-get clean

C) Use gparted after booting from a Live USB and adjust partition sizes to increase the space available to root.