xdiskusage not showing disk space properly in some cases

[Andy Fingerhut]

Using this command shows lots of disk space used in the /snap directory, and subdirectories:

$ du /snap | xdiskusage

Using this command shows nearly 0 disk space (only 4 KiB):

$ xdiskusage /snap

This is not unique to the /snap directory -- it occurs for most directories, maybe all?

$ lsb_release -rd
Description: Ubuntu 18.04.3 LTS
Release: 18.04

$ apt-cache policy xdiskusage
xdiskusage:
  Installed: 1.48-10.1build1
  Candidate: 1.48-10.1build1
  Version table:
 *** 1.48-10.1build1 500
        500 http://us.archive.ubuntu.com/ubuntu bionic/universe amd64 Packages
        100 /var/lib/dpkg/status

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: xdiskusage 1.48-10.1build1
ProcVersionSignature: Ubuntu 4.15.0-62.69-generic 4.15.18
Uname: Linux 4.15.0-62-generic x86_64
ApportVersion: 2.20.9-0ubuntu7.7
Architecture: amd64
CurrentDesktop: ubuntu:GNOME
Date: Sat Sep 14 12:39:09 2019
InstallationDate: Installed on 2018-09-24 (355 days ago)
InstallationMedia: Ubuntu 18.04.1 LTS "Bionic Beaver" - Release amd64 (20180725)
ProcEnviron:
 TERM=xterm-256color
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=en_US.UTF-8
 SHELL=/bin/bash
SourcePackage: xdiskusage
UpgradeStatus: No upgrade log present (probably fresh install)

[Andy Fingerhut (andy-fingerhut)]

It is definitely not most directories that cause this behavior -- only a few. Another example I have found is /mnt

[Andy Fingerhut (andy-fingerhut)]

Ugh, sorry. Scratch my previous comment about the /mnt directory exhibiting this behavior -- I have not observed that. I have observed this difference in behavior between du and xdiskusage with the /media directory, though. And those are the only two top-level directories on my Ubuntu 18.04.3 system that I have observed this behavior with, after trying them all.

[Andy Fingerhut (andy-fingerhut)]

OK, debugging this more myself, I have determined that "xdiskusage <filename>" executes the du program with the -x option, which is documented to "skip directories on different file systems". The /media and /snap directories on my system contain subdirectories that are apparently mounted as different file systems, so that seems to explain the existing behavior.

Sorry for the noise.