Arch Linux

pointerIs404

Member

Registered: 2025-09-15

Posts: 9

[SOLVED] Btrfs Incoherent Disk Usage

My spec:

OS: Arch Linux x86_64  
Host: 20FAS0FD02 (ThinkPad T460s)  
Kernel: Linux 6.16.5-zen1-1-zen  
Packages: 889 (pacman)  
Shell: fish 4.0.2  
WM: Hyprland 0.50.1 (Wayland)  
Memory: 2.84 GiB / 19.38 GiB (15%)  
Swap: 0 B / 17.50 GiB (0%)  
Disk (/): 62.96 GiB / 64.45 GiB (98%) – btrfs  
Disk (/docker): 183.61 MiB / 64.45 GiB (0%) – btrfs  
Disk (/home): 78.18 GiB / 99.80 GiB (78%) – btrfs  

I’m dropping by to say hi - and also because I’m stuck on a problem that’s been bugging me for a while.

This is my second installation. I carefully picked my desktop stack and repartitioned the drive this time. Everything works fine, yet I can’t help but notice an anomaly.

I partitioned my drive as Btrfs with compression set to `zstd:3`. Files are smaller Thunar shows a difference between size and size on disk.

But here’s the weird part: my drive fills up faster than it should.

I did the math (checking every file’s size and summing them). It doesn’t add up. For example, the contents of `/` in the worst case should take around 30–45 GiB, yet the partition shows as completely full. And when I delete big things (e.g. Flatpak libraries or heavy models I stored with Ollama), `/` still reports as full.

At first, I thought this was just the cost of snapshots (I keep system snapshots in case I mess something up). But when I check them, they’re tiny - at most \~100 KiB.

I can’t make sense of it. What is eating my drive ?

Last edited by pointerIs404 (2025-09-16 19:50:16)

Head_on_a_Stick

Member

From: The Wirral

Registered: 2014-02-20

Posts: 9,003

Website

Re: [SOLVED] Btrfs Incoherent Disk Usage

You should use the tools provided by the btrfs devs to analyse disk usage, for example:

btrfs filesystem df /

https://wiki.archlinux.org/title/Btrfs# … free_space

Snapshots will take up more space over time as the state of the running system diverges from the copy.

EDIT: hi!

Last edited by Head_on_a_Stick (2025-09-16 18:33:31)

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Jin, Jîyan, Azadî

pointerIs404

Member

Registered: 2025-09-15

Posts: 9

Re: [SOLVED] Btrfs Incoherent Disk Usage

Thanks, you were right. Even though they appeared as small my snapshot were quite big, as the moment I suppressed half of em I had now 20gb free. plus i discovered on the limitation of  btrfs's so-called copy on write as it seem to eat my drive more it save it.

EDIT: section 7.7 of https://wiki.archlinux.org/title/Btrfs# … free_space was useful

Last edited by pointerIs404 (2025-09-16 19:53:27)