Comment # 2 on bug 957198 from Jason Mader

(In reply to Bernhard Wiedemann from comment #1)
> could it be that the subvolume is mounted?
> or any information in dmesg|tail

Yes, /opt is mounted. The error is different when /opt is not mount,

# btrfs subvolume delete opt
ERROR: 'opt' is not a subvolume

There isn���t anything in dmesg.

getcwd("/", 4096)                       = 2
lstat("/opt", {st_mode=S_IFDIR|0755, st_size=28, ...}) = 0
stat("/", {st_mode=S_IFDIR|0755, st_size=176, ...}) = 0
openat(AT_FDCWD, "/", O_RDONLY|O_NONBLOCK|O_DIRECTORY|O_CLOEXEC) = 3
fstat(1, {st_mode=S_IFCHR|0620, st_rdev=makedev(136, 0), ...}) = 0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) =
0x7f6a1ddf6000
write(1, "Delete subvolume (no-commit): '/"..., 38Delete subvolume (no-commit):
'//opt'
) = 38
ioctl(3, BTRFS_IOC_SNAP_DESTROY, 0x7ffd5c2d2770) = -1 EINVAL (Invalid argument)
write(2, "ERROR: cannot delete '//opt' - I"..., 48ERROR: cannot delete '//opt'
- Invalid argument
) = 48
brk(0xd42000)                           = 0xd42000
close(3)                                = 0