Re: Mysterious device (id 0).

Austin S Hemmelgarn <ahferroin7@xxxxxxxxx> · Fri, 15 May 2015 07:25:01 -0400

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.

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