And also, I'll argue this might have been a btrfs-progs bug as well, depending on what version was used and the command. Both mkfs and dev add should not be able to add type code 0x05. At least libblkid correctly shows that it's 1KiB in size, so really Btrfs should not succeed at adding this device, it can't put any of the supers in the correct location. [chris@f28h ~]$ sudo fdisk -l /dev/loop0 Disk /dev/loop0: 1 GiB, 1073741824 bytes, 2097152 sectors Units: sectors of 1 * 512 = 512 bytes Sector size (logical/physical): 512 bytes / 512 bytes I/O size (minimum/optimal): 512 bytes / 512 bytes Disklabel type: dos Disk identifier: 0x7e255cce Device Boot Start End Sectors Size Id Type /dev/loop0p1 2048 206847 204800 100M 83 Linux /dev/loop0p2 206848 411647 204800 100M 83 Linux /dev/loop0p3 411648 616447 204800 100M 83 Linux /dev/loop0p4 616448 821247 204800 100M 5 Extended /dev/loop0p5 618496 821247 202752 99M 83 Linux [chris@f28h ~]$ sudo kpartx -a /dev/loop0 [chris@f28h ~]$ lsblk NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT loop0 7:0 0 1G 0 loop ├─loop0p1 253:1 0 100M 0 part ├─loop0p2 253:2 0 100M 0 part ├─loop0p3 253:3 0 100M 0 part ├─loop0p4 253:4 0 1K 0 part └─loop0p5 253:5 0 99M 0 part [chris@f28h ~]$ sudo mkfs.btrfs /dev/loop0p4 btrfs-progs v4.17.1 See http://btrfs.wiki.kernel.org for more information. probe of /dev/loop0p4 failed, cannot detect existing filesystem. ERROR: use the -f option to force overwrite of /dev/loop0p4 [chris@f28h ~]$ sudo mkfs.btrfs /dev/loop0p4 -f btrfs-progs v4.17.1 See http://btrfs.wiki.kernel.org for more information. ERROR: mount check: cannot open /dev/loop0p4: No such file or directory ERROR: cannot check mount status of /dev/loop0p4: No such file or directory [chris@f28h ~]$ I guess that's a good sign in this case? Chris Murphy
