I just did:
# btrfs subvolume create /@srv
# btrfs subvolume list / ID 257 gen 73 top level 5 path @ ID 258 gen 71 top level 5 path @home ID 260 gen 65 top level 257 path @srv
And added to /etc/fstab, like this:
/dev/mapper/fs--1--vg-root /home btrfs defaults,compress=zlib,subvol=@home 0 2 /dev/mapper/fs--1--vg-root /srv btrfs defaults,compress=zlib,subvol=@srv 0 2
...where /home is the default that come from Ubuntu's installation and /srv is the one that I manually added, based on what I'm seeing "as an example (i.e., /home)".
But it does not mount, look:
# mount /srv mount: mount(2) failed: No such file or directory
# file /srv /srv: directory
What am I missing?
BTW, the subvolumes @ and @home are okay. But I am unable to create more subvolumes and mount it, side by side with, for example, @home.