Some #btrfs features may make it difficult to estimate what #disk #space is being used for:

#Subvolumes allow cheap copy-on-write #snapshots of entire #filesystem trees, with unmodified data being shared among snapshots
File and extent cloning allow creating cheap copies of files or parts thereof, with extents being stored only once
Compression transparently allows further reducing disk usage

For these reasons, classic disk usage analyzers such as ncdu cannot provide an accurate depiction of actual disk usage. (btrfs compression in particular is challenging to classic analyzers, and special tools must be used to query compressed usage.)

https://github.com/CyberShadow/btdu

GitHub - CyberShadow/btdu: sampling disk usage profiler for btrfs

sampling disk usage profiler for btrfs. Contribute to CyberShadow/btdu development by creating an account on GitHub.

^GitHub