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I've noticed that there is a huge difference in free disk space for root by using "ncdu" or "df" for me:

df -Th:

    dev              devtmpfs  7,8G       0  7,8G    0% /dev
    run              tmpfs     7,8G    1,4M  7,8G    1% /run
    /dev/nvme0n1p2   ext4       25G     19G  4,3G   82% /
    tmpfs            tmpfs     7,8G     77M  7,7G    1% /dev/shm
    tmpfs            tmpfs     7,8G       0  7,8G    0% /sys/fs/cgroup
    tmpfs            tmpfs     7,8G     50M  7,7G    1% /tmp
    /dev/loop1       squashfs  4,3M    4,3M     0  100% /var/lib/snapd/snap/ngrok/11
    /dev/loop0       squashfs   91M     91M     0  100% /var/lib/snapd/snap/core/6405
    /dev/nvme0n1p1   vfat      246M     56M  191M   23% /boot
    /dev/mapper/home ext4      428G     52G  354G   13% /home
    tmpfs            tmpfs     1,6G     20K  1,6G    1% /run/user/

ncdu

10,5 GiB [##        ] /usr
 3,5 GiB [          ] /var
72,0 MiB [          ] /opt
55,1 MiB [          ] /boot
45,9 MiB [          ] /tmp

"ndcu" shows around 5.5gb more free space for root compared to "df". Am I missing something or why the big difference? Thanks in advance!

sleepi
  • 23

2 Answers2

2

Possible scenario

There are various other filesystems mounted under various mountpoints which are (naturally) somewhere deeper under /. If the root filesystem contains files in those places, df will still "count" them (because it queries the filesystem itself) but du won't (because it scans the available tree).

Example

Just after / is mounted, /home contains 4 GB of data. Then another filesystem gets mounted at /home, it contains 52 GB of data. In this case:

  • The root filesystem just reports its own usage to df, this includes those 4 GB.
  • ncdu scans the directory tree. In /home it can see 52 GB from the other filesystem; ncdu -x skips this entirely. In both cases you ignore /home anyway, because you know it's another filesystem.

In the result, after you add up all the relevant lines from ncdu, you will still miss those 4 GB.

Confirmation

Bind mount the same filesystem elsewhere (note -B doesn't mount submounts):

sudo mount -B / /mnt/elsewhere

Analyze what ncdu /mnt/elsewhere yields. Is there data in /mnt/elsewhere/home, /mnt/elsewhere/boot or in another directory that normally is covered by a submount? Can it explain the discrepancy?

Before you remove anything, make sure you know what you're doing. Finally run sudo umount /mnt/elsewhere to unmount.

Kamil Maciorowski
  • 81,893
0

df -Th shows you the "available" disk space. Even when run as root, this is the free space without the (usually ~5%) disk space that is reserved for the root user.

ncdu, when run as root, will take into consideration all the actual free space on the disk, including the space reserved for root. That's why it will report more "free" space than df reports "available" space.

More information on that reserved space and how to change its size can be found in answers to my question here.

confetti
  • 2,605