====== BTRFS Issues ======

===== convert to single with failed drive =====


<code bash>
sudo btrfs balance start -dconvert=single -mconvert=single /media
</code>

Remove drive:
<code bash>sudo btrfs device remove 2</code>


After conversion BTRFS is unmountable. It complains that devices are still missing, even with ''-o degraded,recovery,ro''.
Everything was performed with recent 5.9 kernel.

<code>
BTRFS error (device sdb): super_num_devices 2 mismatch with num_devices 1 found here
BTRFS error (device sdc): failed to read chunk tree: -22
</code>

Solution:
  * [[https://www.mattzone.com/btrfs/btrfs_20171205.html]]
  * Source code of ''fix-dev.count.c'' {{ :linux:fs:btrfs:fix-dev-count.c.gz |}}
References:
  * [[https://www.suse.com/support/kb/doc/?id=000018769|How to recover from BTRFS errors]]
  * [[https://yhbt.net/lore/all/20161012121603.GP11398@twin.jikos.cz/T/|Btrfs unmountable - claims it is missing a volume]]
  * [[https://bbs.archlinux.org/viewtopic.php?id=182505|btrfs partition is suddenly unmountable [SOLVED don't use btrfs]]]
  * [[https://marc.merlins.org/perso/btrfs/post_2014-03-19_Btrfs-Tips_-Btrfs-Scrub-and-Btrfs-Filesystem-Repair.html|You mostly don't need fsck on btrfs]]