this post was submitted on 08 Oct 2025
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(SOLVED) Where did all my space go? (lemmy.world)
submitted 5 days ago* (last edited 2 days ago) by Jozzo@lemmy.world to c/linux@lemmy.ml
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Edit: Solved!

Unfortunately I'm not sure exactly what fixed it, because I was running btrfs commands like a madman. Some combination of the following caused my 100GB labelled as UNREACHABLE to turn into UNUSED, which allowed that space to be written to as normal:

sudo btrfs balance start -v /

sudo btrfs filesystem defrag -v /

sudo btrfs filesystem defrag -v -r /

Also the tool btdu was incredibly helpful!

One of my linux boxes ran out of disk space, which surprised me, because it definitely didn't have that much stuff on it. When I check with df it says I have used 212GB on my / path:

$ df -h /
Filesystem      Size  Used Avail Use% Mounted on
/dev/sda1       227G  212G  5.2G  98% /

So, I tried to use du to see if maybe a runaway log file was the cause, but this says I have only used 101GB on my / path (this is also more in-line with how much space I expected to be used):

$ du -h | sort -h
...
101G    /

Using those commands with sudo outputs the same sizes.

My filesystem is Btrfs, I've tried the suggestion to use btrfs balance start ... but this actually INCREASED my disk usage to 99% lol

So my question is... what on earth is using the remaining 111GB?? Why can I not see it in du?

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[–] Corngood@lemmy.ml 16 points 5 days ago (1 children)

btdu is an excellent tool for finding out what's taking up space in btrfs

[–] Jozzo@lemmy.world 5 points 4 days ago (3 children)

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)

[–] Oinks@lemmy.blahaj.zone 2 points 3 days ago* (last edited 3 days ago) (1 children)

I'm not a btrfs expert but AFAIK high unreachable space usage is usually a result of fragmentation. You might want to defragment the filesystem and see if that helps.

I will note that btrfs makes estimations of used/available space very difficult by design, and you especially can not trust what standard UNIX tools like df and du tell you about btrfs volumes. Scripting around du or using ncdu will not help here in any way. You might want to read this kernel.org wiki article as well as the man pages for the btrfs tools (btrfs(8) and particularly btrfs-filesystem(8)), which among other things provide versions of df and du that actually work, or at least they do most of the time instead of never.

[–] Jozzo@lemmy.world 1 points 2 days ago

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

[–] Corngood@lemmy.ml 1 points 3 days ago

Strange, I've never seen that. Have you rebooted the system to make sure it has nothing to do with open files?

I did find one thread that seems related:

https://www.reddit.com/r/btrfs/comments/lip3dk/unreachable_data_on_btrfs_according_to_btdu/

[–] BCsven@lemmy.ca 3 points 4 days ago (1 children)

There could be btrfs stuff I'm not aware of, but on a file system structure level, do you have a separate drive for booting and then another you added and mounted separately? Or did you install Linux over another install and changed partitions used? The reason I'm asking is you could have a whole drive of data under a folder and then later mount another partition or drive to that same folder. Linux will show you the mounted folder contents, but the original is not visible until you unmount your Mount point. The data is still there. So drive can be full, even though contents look smaller.

I can't say its that for sure, but it has tripped people up before.

But could be btrfs cleanup needs looking at.

[–] Jozzo@lemmy.world 2 points 4 days ago (2 children)

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

[–] magikmw@piefed.social 1 points 4 days ago

I sometimes use btrf assistant, it's a gui helper for btrfs, shows some helpful hints, there's balancing, scrubbing, snapshot cleanup. I should learn more about it all, but it helped a ton with figuring out what's going on with all my space.

[–] BCsven@lemmy.ca 1 points 4 days ago

Yeah, I haven't dealt with that much since OpenSUSE has all the cleanup and maintenance scheduled.

Is your whole drive BTRFS even your /Home on same partition? The BTRFS subvolumes share a pool if setup as one, and filling up home subvolume also fills up your system subvolume, since its shared.