On 2015-05-14 20:09, Christian wrote:
> 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.
>
Have you ever run 'btrfs replace' to replace a device in the filesystem? 
  I know that new device in a replace operation get's tagged as device 
id 0 until the replace operation completes (at which point it gets the 
devid of the device it replaced).  Personally, this is the only case 
that I know of that devid 0 is actually used in the code.  Thinking 
further though, it might also be an off-by-one error somewhere, although 
I have no idea where.