# btrfs subvolume delete /var/snap/lxd/common/lxd/storage-pools/default/containers-snapshots/ntp-backup/working
Delete subvolume (no-commit): '/var/snap/lxd/common/lxd/storage-pools/default/containers-snapshots/ntp-backup/working'
ERROR: cannot delete '/var/snap/lxd/common/lxd/storage-pools/default/containers-snapshots/ntp-backup/working': Operation not permitted

# btrfs subvolume list /var/snap/lxd/common/lxd/storage-pools/default/containers-snapshots/ntp-backup/working
ID 420 gen 2712846 top level 5 path snap/lxd/common/lxd/storage-pools/default
ID 421 gen 2712846 top level 420 path snap/lxd/common/lxd/storage-pools/default/containers
ID 422 gen 2712961 top level 420 path snap/lxd/common/lxd/storage-pools/default/containers-snapshots
ID 423 gen 2712846 top level 420 path snap/lxd/common/lxd/storage-pools/default/images
ID 424 gen 2712846 top level 420 path snap/lxd/common/lxd/storage-pools/default/custom
ID 425 gen 2712846 top level 420 path snap/lxd/common/lxd/storage-pools/default/custom-snapshots
ID 510 gen 2643133 top level 422 path snap/lxd/common/lxd/storage-pools/default/containers-snapshots/ntp-backup/working

# btrfs subvolume show /var/snap/lxd/common/lxd/storage-pools/default/containers-snapshots/ntp-backup/working
snap/lxd/common/lxd/storage-pools/default/containers-snapshots/ntp-backup/working
        Name:                   working
        UUID:                   6e3f2cbf-14fc-2742-8ff5-a24ce137966a
        Parent UUID:            a2159e66-f131-6c46-907f-7958cce550a2
        Received UUID:          -
        Creation time:          2019-05-15 09:37:15 +0200
        Subvolume ID:           510
        Generation:             2643133
        Gen at creation:        2643133
        Parent ID:              422
        Top level ID:           422
        Flags:                  readonly
        Snapshot(s):

# btrfs property set -ts /var/snap/lxd/common/lxd/storage-pools/default/containers-snapshots/ntp-backup/
working ro false
ERROR: failed to set flags for /var/snap/lxd/common/lxd/storage-pools/default/containers-snapshots/ntp-backup/working: Operation not permitted

# uname -r
4.15.0-51-generic