Mysterious device (id 0).

[Christian <cdysthe@gmail.com>]

Hi,

My laptop has a small ext4 /boot partition /dev/sda1 and a large btrfs 
partition containing / and /home as a sub volume /dev/sda3

When I run scrub on /dev/sda3 and check status with sudo btrfs scrub 
status -d /dev/sda3 I get:

scrub status for 3d52dc93-c89f-453f-965d-8601d11e7710
scrub device /dev/sda3 (id 1) history
	scrub started at Thu May 14 19:54:02 2015 and finished after 235 seconds
	total bytes scrubbed: 111.03GiB with 0 errors
scrub device  (id 0) history
	scrub started at Thu May 14 19:54:02 2015 and was aborted after 0 seconds
	total bytes scrubbed: 0.00B with 0 errors

As device (id 0) is indicated, but I have no idea what it is.

When I run scrub with a script (cron) the same device (id 0) shows up in 
a warning:

/etc/cron.daily/btrfs-scrub:
WARNING: device 0 not present
scrub device /dev/sda3 (id 1) done
     scrub started at Sat May  2 08:03:46 2015 and finished after 241 
seconds
     total bytes scrubbed: 109.06GiB with 0 errors
scrub device  (id 0) canceled
     scrub started at Sat May  2 08:03:46 2015 and was aborted after 0 
seconds
     total bytes scrubbed: 0.00B with 0 errors

What is this device (id 0) which never gets scrubbed but always shows up?

I'm running Ubuntu 15.04 with the 3.19.0-16-generic kernel.

P.S. I am not a developer.

-- 
//Christian