What could be the source of drastically different disk usage reports between df, lsblk and baobab?

Question

Disk Usage Analyzer (baobab) reports a size of 29.3 GB for the root partition, out of which 2.5 GB are available:

However, when selecting that device for closer inspection, I am presented with only 7.9 GB of stored data:

There are further inconsistencies between different disk usage tools:

1.  > df -h
    Filesystem      Size  Used Avail Use% Mounted on
    udev            7.7G     0  7.7G   0% /dev
    tmpfs           1.6G  2.2M  1.6G   1% /run
    /dev/nvme0n1p2   28G   26G  940M  97% /              # <- ~30GB root partition
    tmpfs           7.7G     0  7.7G   0% /dev/shm
    tmpfs           5.0M  8.0K  5.0M   1% /run/lock
    /dev/nvme0n1p4  440G  171G  248G  41% /home
    /dev/nvme0n1p1   96M   50M   47M  52% /boot/efi
    tmpfs           1.6G  2.5M  1.6G   1% /run/user/1000
   

2.  > lsblk -f
   
    NAME        FSTYPE FSVER LABEL UUID                                 FSAVAIL FSUSE% MOUNTPOINTS
    nvme0n1                                                                            
    ├─nvme0n1p1 vfat   FAT32       14EC-1EA1                              46.1M    52% /boot/efi
    ├─nvme0n1p2 ext4   1.0         6b28d7fa-8554-48df-a1f0-a4a8e7ccfc6e  939.7M    91% /        # <- ~10 GB root partition
    ├─nvme0n1p3 swap   1           e3b8188b-ab13-4d5c-b5fd-e23440f816f1                [SWAP]
    └─nvme0n1p4 ext4   1.0         a8fbcfb2-70ff-4474-8475-70b948bbc557  247.1G    39% /home
   

   > lsblk -o NAME,FSAVAIL,FSUSE%,SIZE,MOUNTPOINTS
   
   NAME        FSTYPE FSAVAIL FSUSE%   SIZE MOUNTPOINTS
   nvme0n1                           476.9G 
   ├─nvme0n1p1 vfat     46.1M    52%   100M /boot/efi
   ├─nvme0n1p2 ext4    939.7M    91%  27.9G /             # ~10 GB or ~ 30 GB root partition?
   ├─nvme0n1p3 swap                    977M [SWAP]
   └─nvme0n1p4 ext4    247.1G    39% 447.9G /home
   

3.  > sudo dumpe2fs -h /dev/nvme0n1p2 | grep Block
    dumpe2fs 1.47.0 (5-Feb-2023)
    Block count:              7324160
    Block size:               4096
    Blocks per group:         32768
   

Multiplying block count by block size gives ~29GB for the filesystem size.

In summary:

• baobab reports roughly 27GB out of 29 GB space used, while simultaneously showing only 7.9 GB being used.
• df reports 26 out of 28GB being used (1).
• lsblk reports a SIZE of 27.9G but at the same time claims that the 939.7M available space (FSAVAIL) corresponds to 9% (100% minus FSUSE%) of the device, which would give a size of roughly 10 GB (2).
• dumpe2fs reports a filesystem size of almost 30GB (3).

---

Overall there seem to be two conflicting pictures, represented by different tools:

• The partition and filesystem have a size of roughly 30GB and are nearly full.
• The partition and filesystem have a size of roughly 10GB and are nearly full.

What could be the source for this discrepancy?

Answer 1

Thanks to @frostschutz' comment and link to a related question, the problem could be further diagnosed using a bind mount:

mkdir /mnt/root
mount --bind / /mnt/root

Upon inspection of the filesystem now under /mnt/root/, it turns out that there were lots of files in the root partition under /home/kevin/ that were inaccessible because the home partition was mounted at /home/kevin/.

These hidden files were the source of the discrepancy.