nas brot # mkfs.btrfs -d raid1 -m raid1 /dev/sdd3 /dev/sde3

nas brot # btrfs filesystem show /dev/sdd3
Label: none  uuid: c356873f-4034-43cc-8052-b2a4bf68cb26
        Total devices 2 FS bytes used 112.00KiB
        devid    1 size 4.55TiB used 2.03GiB path /dev/sdd3
        devid    2 size 4.55TiB used 2.01GiB path /dev/sde3

Btrfs v3.17

nas brot # mount /dev/sdd3 /mnt/btrfs_rootsubvol/

nas brot # btrfs subvolume create /mnt/btrfs_rootsubvol/root-subvol
Create subvolume '/mnt/btrfs_rootsubvol/root-subvol'
nas brot # btrfs subvolume create /mnt/btrfs_rootsubvol/data-subvol
Create subvolume '/mnt/btrfs_rootsubvol/data-subvol'
nas brot # btrfs subvolume create /mnt/btrfs_rootsubvol/backup-subvol
Create subvolume '/mnt/btrfs_rootsubvol/backup-subvol'
nas brot # btrfs subvolume create /mnt/btrfs_rootsubvol/brot-backup-subvol
Create subvolume '/mnt/btrfs_rootsubvol/brot-backup-subvol'

nas brot # btrfs fi df /mnt/btrfs_rootsubvol/
Data, RAID1: total=1.00GiB, used=512.00KiB
Data, single: total=8.00MiB, used=0.00B
System, RAID1: total=8.00MiB, used=16.00KiB
System, single: total=4.00MiB, used=0.00B
Metadata, RAID1: total=1.00GiB, used=176.00KiB
Metadata, single: total=8.00MiB, used=0.00B
GlobalReserve, single: total=16.00MiB, used=0.00B

nas ~ # btrfs subvolume list /mnt/btrfs_rootsubvol/
ID 258 gen 155 top level 5 path root-subvol
ID 259 gen 8 top level 5 path data-subvol
ID 260 gen 9 top level 5 path backup-subvol
ID 261 gen 10 top level 5 path brot-backup-subvol

nas ~ # btrfs subvolume set-default 258 /mnt/btrfs_rootsubvol/
(setzen vom rootfs als std-subvol)