Re: Recovery from full metadata with all device space consumed?

[Drew Bloechl <drew@cesspool.net>]

On Thu, Apr 19, 2018 at 10:43:57PM +0000, Hugo Mills wrote:
>    Given that both data and metadata levels here require paired
> chunks, try adding _two_ temporary devices so that it can allocate a
> new block group.

Thank you very much, that seems to have done the trick:

# fallocate -l 4GiB /var/tmp/btrfs-temp-1
# fallocate -l 4GiB /var/tmp/btrfs-temp-2
# losetup -f /var/tmp/btrfs-temp-1
# losetup -f /var/tmp/btrfs-temp-2
# btrfs device add /dev/loop0 /broken
Performing full device TRIM (4.00GiB) ...
# btrfs device add /dev/loop1 /broken
Performing full device TRIM (4.00GiB) ...
# btrfs balance start -v -dusage=1 /broken
Dumping filters: flags 0x1, state 0x0, force is off
  DATA (flags 0x2): balancing, usage=1

I'm guessing that'll take a while to complete, but meanwhile, in another
terminal:

# btrfs fi show /broken
Label: 'mon_data'  uuid: 85e52555-7d6d-4346-8b37-8278447eb590
	Total devices 6 FS bytes used 69.53GiB
	devid    1 size 931.51GiB used 731.02GiB path /dev/sda1
	devid    2 size 931.51GiB used 731.02GiB path /dev/sdb1
	devid    3 size 931.51GiB used 730.03GiB path /dev/sdc1
	devid    4 size 931.51GiB used 730.03GiB path /dev/sdd1
	devid    5 size 4.00GiB used 1.00GiB path /dev/loop0
	devid    6 size 4.00GiB used 1.00GiB path /dev/loop1

# btrfs fi df /broken
Data, RAID0: total=2.77TiB, used=67.00GiB
System, RAID1: total=8.00MiB, used=192.00KiB
Metadata, RAID1: total=4.00GiB, used=2.49GiB
GlobalReserve, single: total=512.00MiB, used=0.00B

Do I understand correctly that this could require up to 3 extra devices,
if for instance you arrived in this situation with a RAID6 data profile?
Or is the number even higher for profiles like RAID10?