When trying to repair a btrfs root filesystem I get the following error message: # losetup -f /home/fturco/Buffer/root-20160616.img # btrfs check --repair /dev/loop0 enabling repair mode warning, device 4 is missing Checking filesystem on /dev/loop0 UUID: 34fb5b58-f50f-47c3-a5b8-91d81a30eade checking extents Unable to find block group for 0 extent-tree.c:289: find_search_start: Assertion `1` failed. btrfs[0x44882e] btrfs(btrfs_reserve_extent+0xaa9)[0x44d639] btrfs(btrfs_alloc_free_block+0x5f)[0x44d6ff] btrfs(__btrfs_cow_block+0xc4)[0x43e9d4] btrfs(btrfs_cow_block+0x35)[0x43efd5] btrfs[0x4442e6] btrfs(btrfs_commit_transaction+0x95)[0x446115] btrfs[0x42b58e] btrfs(cmd_check+0x76c)[0x42c60c] btrfs(main+0x7b)[0x40ac3b] /usr/lib/libc.so.6(__libc_start_main+0xf1)[0x7f272f658741] btrfs(_start+0x29)[0x40ad39] This is an unmountable filesystem: # mount /dev/loop0 /mnt/ mount: wrong fs type, bad option, bad superblock on /dev/loop0, missing codepage or helper program, or other error In some cases useful info is found in syslog - try dmesg | tail or so. # dmesg | tail [ 9185.503571] scsi 8:0:0:1: Direct-Access Samsung File-CD Gadget 0000 PQ: 0 ANSI: 2 [ 9185.513943] sd 8:0:0:0: [sdb] Attached SCSI removable disk [ 9185.514565] sd 8:0:0:1: [sdc] Attached SCSI removable disk [13773.533435] usb 1-4: USB disconnect, device number 8 [31454.672925] loop: module loaded [31455.600069] BTRFS: device fsid 34fb5b58-f50f-47c3-a5b8-91d81a30eade devid 1 transid 81460 /dev/loop0 [32584.640398] BTRFS info (device loop0): disk space caching is enabled [32584.640403] BTRFS: has skinny extents [32584.654196] BTRFS: failed to read the system array on loop0 [32584.666774] BTRFS: open_ctree failed # btrfs filesystem show /dev/loop0 warning, device 4 is missing Label: none uuid: 34fb5b58-f50f-47c3-a5b8-91d81a30eade Total devices 2 FS bytes used 5.17GiB devid 1 size 30.00GiB used 30.00GiB path /dev/loop0 *** Some devices missing If I remember correctly I extended that root filesystem with some additional space from a file in the home directory, in the hope of fixing a problem with btrfs balance and not enough space. I don't have the additional file anymore, so I probably won't be able to mount this image file anymore. Anyway btrfs-check shouldn't crash. I'm running Parabola GNU/Linux-libre with the following package versions: - linux-libre: 4.6.2 - btrfs-progs: 4.5.3