Looks like some combination of defragging & balancing has done the trick! The space that was previously marked UNREACHABLE is now UNUSED, and my disk space is back to normal:

Filesystem      Size  Used Avail Use% Mounted on
/dev/sda1       227G  105G  103G  51% /

Thanks for the wiki link, Btrfs is new to me and I’ve definitely got some learning to do

Just one drive, it was a clean Linux install with no funky stuff going on. I’ll have to look into Btrfs cleanup more, last time I did it the disk just filled up even more

Legend! It found a second filesystem named “UNREACHABLE”:

It looks like an exact duplicate of my main filesystem “/@rootfs”, I’m guessing this is why my disk space filled up. Do you know how I’d go about removing the duplicate? (If it’s safe to do so)

Interesting, this could be it? I haven’t configured any mounts on this device yet, but when I tried one of the other suggestions from this thread and use btdu, I get this error:

$ ./btdu -x /
Fatal error: The mount point you specified, "/", is not the top-level btrfs subvolume ("subvolid=5,subvol=/").
It is the btrfs subvolume "subvolid=256,subvol=/@rootfs".
Please specify the path to a mountpoint mounted with subvol=/ or subvolid=5.
E.g.: mkdir /mnt/sda1 && mount -o subvol=/ /dev/sda1 /mnt/sda1 && ./btdu /mnt/sda1

Note that the top-level btrfs subvolume ("subvolid=5,subvol=/") is not the same as the root of the filesystem ("/").

I’m fairly new to the workings of Btrfs so this is jibberish to me right now, but I’ll look into it more

EDIT: Nevermind! I was just using the tool wrong. I needed to mount my btrfs “sub-volume” then do the scan against that:

sudo mkdir -p /mnt/btdu

sudo mount -o subvolid=5 /dev/sda1 /mnt/btdu

sudo ./btdu /mnt/btdu

ncdu

Oh this one is very cool! Unfortunately it also only shows the same 101GB being used:

ncdu 1.22 ~ Use the arrow keys to navigate, press ? for help                                                                                                                                  
--- / ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
   93.1 GiB [###########################] /home                                                                                                                                               
    6.5 GiB [#                          ] /usr
  790.4 MiB [                           ] /var
  173.0 MiB [                           ] /boot
   12.8 MiB [                           ] /etc
    1.7 MiB [                           ] /root
    1.3 MiB [                           ] /run
   44.0 KiB [                           ] /tmp
@   4.0 KiB [                           ]  initrd.img.old
@   4.0 KiB [                           ]  initrd.img
@   4.0 KiB [                           ]  vmlinuz.old
@   4.0 KiB [                           ]  vmlinuz
@   4.0 KiB [                           ]  lib64
@   4.0 KiB [                           ]  sbin
@   4.0 KiB [                           ]  lib
@   4.0 KiB [                           ]  bin
.   0.0   B [                           ] /proc
    0.0   B [                           ] /sys
    0.0   B [                           ] /dev
    0.0   B [                           ] /media
e   0.0   B [                           ] /srv
e   0.0   B [                           ] /opt
e   0.0   B [                           ] /mnt

There is one listed: ID 256 gen 137604 top level 5 path @rootfs Looks like it is just my filesystem though?

Yep it’s not near finished. This is one of those projects that’s sat in my “keep an eye on it” bookmarks for a good while, I figured I’d post it to get some attention on it, because it does look very promising.