Samuel Sieb wrote:
...
> > compression is set per subvolume.  I don't know why Chris said that. 
> The btrfs docs say it's per mount.

what Chris said seems valid !
just checked it:

1. Test
=======
- *only* removed "compress=zstd:1" from subvolume /home/DATA in my fstab
- rebooted

mount | grep btrfs gives:
/dev/nvme0n1p3 on /home/DATA type btrfs 
(rw,relatime,seclabel,compress=zstd:1,ssd,discard=async,space_cache=v2,subvolid=258,subvol=/DATA)

=>
the subvolume is mounted with compression, even when unset !

2. Test
=======
- addional removed "compress=zstd:1" from / 
- and leave all as is, e.g. subvolume /home has still "compress=zstd:1" set in 
my /etc/fstab
- rebooted

/etc/fstab looks like this:
# for /dev/nvme0n1p3 (a btrfs pool)
...
UUID=2ab6e75b-4e3c-4a9d-a329-431178fdd0e7 /                    btrfs 
subvol=ROOT 0 0
UUID=2ab6e75b-4e3c-4a9d-a329-431178fdd0e7 /home           btrfs 
subvol=HOME,compress=zstd:1 0 0
UUID=2ab6e75b-4e3c-4a9d-a329-431178fdd0e7 /home/DATA btrfs subvol=DATA 0 0

mount | grep btrfs gives: 

/dev/nvme0n1p3 on / type btrfs 
(rw,relatime,seclabel,ssd,discard=async,space_cache=v2,subvolid=256,subvol=/ROOT)
/dev/nvme0n1p3 on /home type btrfs 
(rw,relatime,seclabel,ssd,discard=async,space_cache=v2,subvolid=257,subvol=/HOME)
/dev/nvme0n1p3 on /home/DATA type btrfs 
(rw,relatime,seclabel,ssd,discard=async,space_cache=v2,subvolid=258,subvol=/DATA)

=>
/ and all subvolumes are mouted *without* compression, even when one subvolume 
in the btrfs pool has compression set (is this case: /home)
-- 
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