Re: `btrfs dev del` fails with `No space left on device`

[Hugo Mills <hugo@carfax.org.uk>]

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On Tue, Aug 30, 2016 at 10:22:24AM +0000, ojab // wrote:
> On Mon, Aug 29, 2016 at 9:05 PM, Chris Murphy <lists@colorremedies.com> wrote:
> > On Mon, Aug 29, 2016 at 10:04 AM, ojab // <ojab@ojab.ru> wrote:
> > What do you get for 'btrfs fi us <mp>'
> 
> $ sudo btrfs fi us /mnt/xxx/
> Overall:
>     Device size:                  3.64TiB
>     Device allocated:             1.82TiB
>     Device unallocated:           1.82TiB
>     Device missing:                 0.00B
>     Used:                     1.81TiB
>     Free (estimated):             1.83TiB         (min: 943.55GiB)
>     Data ratio:                  1.00
>     Metadata ratio:                  2.00
>     Global reserve:             512.00MiB         (used: 0.00B)
> 
> Data,RAID0: Size:1.81TiB, Used:1.80TiB
>    /dev/sdb1        928.48GiB
>    /dev/sdc1        928.48GiB
> 
> Metadata,RAID1: Size:3.00GiB, Used:2.15GiB
>    /dev/sdb1          3.00GiB
>    /dev/sdc1          3.00GiB
> 
> System,RAID1: Size:32.00MiB, Used:176.00KiB
>    /dev/sdb1         32.00MiB
>    /dev/sdc1         32.00MiB
> 
> Unallocated:
>    /dev/sdb1          1.01MiB
>    /dev/sdc1          1.00MiB
>    /dev/sdd1          1.82TiB

   Basically, your FS is full. Note that replacing just one of the
devices in a RAID-0 array with a larger one is not going to help. What
you really need here is "single", not RAID-0. I would probably try
converting to single first (which should move quite a lot of it to the
new device anyway), and then the device delete:

 btrfs balance start -dconvert=single,soft /mountpoint
 btrfs dev delete /dev/sdc1 /mountpoint

   Hugo.

> > You can see what the state of block groups are with btrfs-debugfs
> > which is in kdave btrfs-progs git. Chances are you need a larger
> > value, -dusage=15 -musage=15 to free up space on devid 1 and 2. Then
> > maybe devid 3 can be removed.
> 
> btrfs-debugfs output:
> https://gist.github.com/ojab/a3c59983e8fb6679b8fdc0e88c0c9e60
> Before `delete` the was about 60Gb of free space, looks like it was
> filled during `delete` (I've seen similar behavior during `btrfs fi
> defrag`) and I should use `-dusage=69` and up.
> 
> I don't quite understand what exactly btrfs is trying to do: I assume
> that block groups should be relocated to the new/empty drive, but
> during the delete `btrfs fi us` shows
> Unallocated:
> /dev/sdc1         16.00EiB
> 
> so deleted partition is counted as maximum possible empty drive and
> blocks are relocated to it instead of new/empty drive? (kernel-4.7.2 &
> btrfs-progs-4.7.1 here)
> Is there any way to see where and why block groups are relocated
> during `delete`?
> 
> //wbr ojab

-- 
Hugo Mills             | Everything simple is false. Everything which is
hugo@... carfax.org.uk | complex is unusable
http://carfax.org.uk/  |
PGP: E2AB1DE4          |

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