Ceph » ceph-volume

the rook prepare step fails executing `ceph volume inventory /dev/sda`.

```
ceph-volume inventory --format json /dev/sda
Traceback (most recent call last):
File "/usr/sbin/ceph-volume", line 11, in <module>
load_entry_point('ceph-volume==1.0.0', 'console_scripts', 'ceph-volume')()
File "/usr/lib/python3.6/site-packages/ceph_volume/main.py", line 41, in init
self.main(self.argv)
File "/usr/lib/python3.6/site-packages/ceph_volume/decorators.py", line 59, in newfunc
return f(*a, **kw)
File "/usr/lib/python3.6/site-packages/ceph_volume/main.py", line 153, in main
terminal.dispatch(self.mapper, subcommand_args)
File "/usr/lib/python3.6/site-packages/ceph_volume/terminal.py", line 194, in dispatch
instance.main()
File "/usr/lib/python3.6/site-packages/ceph_volume/inventory/main.py", line 56, in main
self.format_report(Device(self.args.path, with_lsm=self.args.with_lsm))
File "/usr/lib/python3.6/site-packages/ceph_volume/util/device.py", line 126, in init
sys_info.devices = disk.get_devices(device=self.path)
File "/usr/lib/python3.6/site-packages/ceph_volume/util/disk.py", line 861, in get_devices
if is_ceph_rbd(diskname):
File "/usr/lib/python3.6/site-packages/ceph_volume/util/disk.py", line 397, in is_ceph_rbd
return dev.startswith(('/dev/rbd'))
AttributeError: 'NoneType' object has no attribute 'startswith'
```

1. lsblk /dev/sda
   NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT
   sda 8:0 0 14.6T 0 disk
   ```