ButterFS, like ZFS but does not eat all of your ram.
Some options I use
rw,noatime,compress-force=zstd:1,ssd,user_subvol_rm_allowed,autodefrag,subvol=@
Kernel cmdline needs only this because systemd will remount the root with correct options anyway
root=LABEL=ROOT rootflags=subvol=@
This was needed on kernels between 5.6 and 6.2. Newer kernels are running async discard by default.
This was needed only on realy old kernels (<4.5). Now defaults to space_cache=v2
Since 6.7 you can enable simple_quota space accounting on subvolumes. It's a lightweight more performant alternative to normal quota accounting.
btrfs quota enable --simple /
Drawback is that simple_quota is aqurate only for subvolumes (not snapshots) and only for data that was written after simple_quota was enabled (there is currently no rescan option like in classic quota system). This works pretty well for figuring out how much disk a systemd-nspawn container uses.
NB: Kernels before 6.11 do not automatically cleanup quota groups left over after a subvolume/snapshot is deleted. You need to manually run btrfs qgroup clear-stale / to clean them up.
NB: Simple quota group removal can trigger a kernel bug if the qgroup size is 0
NB: Hierarical quota also seems to have bugs as I'm not able to delete parent qgroups that use to refference subvolume qgroups
that don't exist any more. (TODO investigate more)
Files created using fallocate(2) break btrfs.
Reproducer:
# fallocate -l 128m data
# compsize .
Processed 1 file, 1 regular extents (1 refs), 0 inline.
Type Perc Disk Usage Uncompressed Referenced
TOTAL 100% 128M 128M 128M
prealloc 100% 128M 128M 128M
# truncate -s 128m data
# compsize .
Processed 1 file, 1 regular extents (1 refs), 0 inline.
Type Perc Disk Usage Uncompressed Referenced
TOTAL 100% 128M 128M 4.0K
prealloc 100% 128M 128M 4.0K
# stat data
File: data
Size: 4096 Blocks: 8 IO Block: 4096 regular file
This mostly problem for databases (PostgreSQL, Prometheus) and torrent clients that by default fallocate all their files.
btrfs subvolume list with btrfs-list https://github.com/speed47/btrfs-listcompsize