I have btrfs compress= mount option in my fstab file probably inserted by Anaconda installer:
LABEL=ssd / btrfs subvol=fedora,compress=zstd:1,ro 0 0
That suggests the compression should be enabled, which is inline with Enable btrfs transparent zstd compression by default Fedora proposal.
But the compression seems actually disabled, probably because / is composefs instead:
$ mount
composefs on / type overlay (ro,relatime,seclabel,lowerdir+=/run/ostree/.private/cfsroot-lower,datadir+=/sysroot/ostree/repo/objects,redirect_dir=on,metacopy=on)
$ mount | grep compress
# empty
$ sudo dmesg | grep -i btrfs
[ 0.658924] Btrfs loaded, zoned=yes, fsverity=yes
[ 4.619621] BTRFS: device label ssd devid 1 transid 59306 /dev/nvme0n1p1 (259:1) scanned by mount (533)
[ 4.619846] BTRFS info (device nvme0n1p1): first mount of filesystem 6f529918-577b-4595-8059-e6fed0197788
[ 4.619871] BTRFS info (device nvme0n1p1): using crc32c (crc32c-lib) checksum algorithm
[ 4.643678] BTRFS info (device nvme0n1p1): enabling ssd optimizations
[ 4.643697] BTRFS info (device nvme0n1p1): turning on async discard
[ 4.643707] BTRFS info (device nvme0n1p1): enabling free space tree
Is it Anaconda/Fedora bug or what?