I just tried. At least I think I did… My /srv/ is mounted to my mSATA disk which is BTRFS formatted (how I currently make manual BTRFS subvolume snapshots form my containers on FS level).
When I simply lxc-snapshot -n <containername> it gives the same output as you posted (despite it being a BTRFS subvolume). So I wanted to make an lxc-clone instead (telling it to be a (btrfs)layered clone) to, in-turn take snapshots from, which is what the instructions tell us to do, and which is what I did before on other (“regular”) GNU/Linux boxes but lxc-clone is missing from the Turris’ lxc toolkit (or can I now - seeing rsync got some attention from the devs - simply install/use lxc-clone now?):
I am more than willing to test stuff, if someone can point me into the right direction and tell me how to do it. Clearly I am missing some experience/knowledge here (maybe I should create a test container “telling” lxc to make use of the BTRFS possibilities?), or maybe the FS MUST be aufs / overlayfs?
Let’s “put our houlders under this one” together, shall we? 