On Wed, 13 May 2026 at 19:49, Giles Orr <gilesorr@gmail.com> wrote:
I have a system with Debian 13 installed. I was installing something on the machine and ran out of space on the root partition (big Docker image). After it aborted the install, here's the output of `df`:
# df -h Filesystem Size Used Avail Use% Mounted on ... /dev/mapper/brick_lvm-os2 24G 21G 2.6G 90% / ...
Here's the output of `ncdu -x /`:
8.4 GiB [#############] /usr
3.2 GiB [#### ] /var 296.6 MiB [ ] /boot 99.5 MiB [ ] /root 19.4 MiB [ ] /etc 16.0 KiB [ ] /media 16.0 KiB [ ] /opt e 16.0 KiB [ ] /lost+found 12.0 KiB [ ] /mnt ... *Total disk usage: 11.9 GiB Apparent size: 11.6 GiB Items: 344362
You see the problem? `ncdu` thinks 12G is used, but `df` thinks that 21G is used. That's one hell of a discrepancy. A web search yielded the idea of running `lsof | grep deleted` which turned up Firefox as a major offender ... but killing FF got rid of its hundreds of "deleted" items while not changing what `df` thought at all.
I remember you can hide disk usage under mount points. But that was an old trick one hundred years ago on multi-user systems. One that I'd be unlikely to play on myself as this isn't a multi-user system.
Other things I checked: no, there isn't a giant swap file sitting in the root. No, this problem doesn't exist on my main machine (or, I think, any of my others). The one unusual thing about this system is that ZFS is installed and there are two other mirrored drives. Root is hosted on a separate drive, so I don't think ZFS would be quietly eating huge chunks of this non-ZFS drive? (I am not a ZFS expert.)
This is an LVM system and I have spare space that I can throw at the problem - but I'd really really like to understand what's going on!
Anybody seen anything like this? Any ideas what would cause this discrepancy or how to track it down? Thanks.
To clarify: - the system has three spinning disks - two are mirrored 6TB ZFS drives - but the boot drive is a separate 1TB drive managed by LVM and the root partition and all of the other partitions on that drive are ext4, not ZFS - I understand there are differences between how `du` and `df` calculate things, but we're looking at a discrepancy of 9G (about 40% of the partition capacity) so it's a stretch for me to believe that this is the problem ... -- Giles https://www.gilesorr.com/ gilesorr@gmail.com