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/tmp # uname -a
Linux tatava 4.9.0-3-amd64 #1 SMP Debian 4.9.30-2+deb9u3 (2017-08-06) x86_64 GNU/Linux
/tmp # mkfs.btrfs -f -m raid1 -d raid1 /dev/loop5 /dev/loop6
btrfs-progs v4.12
See http://btrfs.wiki.kernel.org for more information.

Performing full device TRIM /dev/loop5 (476.84MiB) ...
Performing full device TRIM /dev/loop6 (476.84MiB) ...
Label:              (null)
UUID:               3638fdbe-8049-4d49-accc-d98b65dd9fff
Node size:          16384
Sector size:        4096
Filesystem size:    953.67MiB
Block group profiles:
  Data:             RAID1            64.00MiB
  Metadata:         RAID1            47.62MiB
  System:           RAID1             8.00MiB
SSD detected:       no
Incompat features:  extref, skinny-metadata
Number of devices:  2
Devices:
   ID        SIZE  PATH
    1   476.84MiB  /dev/loop5
    2   476.84MiB  /dev/loop6

/tmp # mount /dev/loop5 /mnt/btrfs/
/tmp # echo ahoj > /mnt/btrfs/ble
/tmp # umount /mnt/btrfs 
/tmp # losetup -d /dev/loop6
/tmp # mount /dev/loop5 /mnt/btrfs/
mount: wrong fs type, bad option, bad superblock on /dev/loop5,
       missing codepage or helper program, or other error

       In some cases useful info is found in syslog - try
       dmesg | tail or so.
[409621.818397] BTRFS info (device loop5): disk space caching is enabled
[409621.818408] BTRFS info (device loop5): has skinny extents
[409621.820543] BTRFS error (device loop5): failed to read chunk tree: -5
[409621.880963] BTRFS error (device loop5): open_ctree failed
/tmp # mount -o degraded /dev/loop5 /mnt/btrfs/
/tmp # dmesg -c
[409633.574482] BTRFS info (device loop5): allowing degraded mounts
[409633.574495] BTRFS info (device loop5): disk space caching is enabled
[409633.574500] BTRFS info (device loop5): has skinny extents
/tmp # echo ahoj2 > /mnt/btrfs/ble2
/tmp # umount /mnt/btrfs 
/tmp # mount -o degraded /dev/loop5 /mnt/btrfs/
/tmp # echo ahoj3 > /mnt/btrfs/ble3
/tmp # umount /mnt/btrfs 
/tmp # mount -o degraded /dev/loop5 /mnt/btrfs/
mount: wrong fs type, bad option, bad superblock on /dev/loop5,
       missing codepage or helper program, or other error

       In some cases useful info is found in syslog - try
       dmesg | tail or so.
[409667.522866] BTRFS info (device loop5): allowing degraded mounts
[409667.522877] BTRFS info (device loop5): disk space caching is enabled
[409667.522881] BTRFS info (device loop5): has skinny extents
[409667.524987] BTRFS warning (device loop5): missing devices (1) exceeds the limit (0), writeable mount is not allowed
[409667.601637] BTRFS error (device loop5): open_ctree failed
/tmp #