I tried to make a btrfs RAID-1 filesystem in "degraded mode" by following the btrfs UseCases instructions but hit a fatal assertion error. Why is this failing, and is there any workaround? The instructions I followed are at: https://btrfs.wiki.kernel.org/articles/u/s/e/UseCases_8bd8.html The output of the mkfs.btrfs and btrfs filesystem show commands is: # mkfs.btrfs -m raid1 -d raid1 /dev/sdd1 /dev/loop1 WARNING! - Btrfs Btrfs v0.19 IS EXPERIMENTAL WARNING! - see http://btrfs.wiki.kernel.org before using failed to read /dev/sr0 adding device /dev/loop1 id 2 mkfs.btrfs: volumes.c:802: btrfs_alloc_chunk: Assertion `!(ret)' failed. zsh: abort (core dumped) mkfs.btrfs -m raid1 -d raid1 /dev/sdd1 /dev/loop1 # btrfs filesystem show failed to read /dev/sr0 Label: none uuid: 773908b8-acca-4c30-85c5-6642b06de22b Total devices 1 FS bytes used 28.00KB devid 1 size 223.13GB used 2.04GB path /dev/sda5 Label: none uuid: 0f06f1a8-5f5f-4b92-a55c-b827bcbcc840 Total devices 2 FS bytes used 24.00KB devid 2 size 2.00GB used 0.00 path /dev/loop1 devid 1 size 1.36TB used 20.00MB path /dev/sdd1 Btrfs Btrfs v0.19 # EDIT: It turns out that the filesystem isn't mountable: # mount /dev/sdd1 /mnt/big2 mount: wrong fs type, bad option, bad superblock on /dev/sdd1, missing codepage or helper program, or other error In some cases useful info is found in syslog - try dmesg | tail or so # So, why did the mkfs fail, and is there any workaround?