### BTRFS RAID-1 (data mirror) quickstart ###
       
       
       I used to rely on mdadm for Linux RAID-1 (mirror) setups - until the day I lost
       data because one of the disks in the RAID started burping corrupted bytes and
       mdadm didn't realized there was an issue.
       
       Nowadays I prefer to use the RAID facility embedded in BTRFS. It might not be
       as fast as mdadm, but I found it to be more reliable, and also less troublesome
       to setup, esp. in configurations where the RAID is used as boot device.
       
       One thing to keep in mind: BTRFS is a filesystem, hence the RAID is not a RAID
       of disks, but a RAID of filesystem containers (typically: partitions). While it
       is possible to write a BTRFS filesystem directly on a block device, there is no
       advantage to do it, only limitations.
       
       Let's assume we have two partitions that we want to bind together in a mirror:
       /dev/sda1 and /dev/sdb1, and the filesystem is mounted at /srv.
       Here below are the commands you should know.
       
       Wipe the filesystem of a device so it can be reformatted with btrfs:
       # wipefs -a /dev/sda1
       
       Creating the mirror RAID:
       # mkfs.btrfs -m raid1 -d raid1 /dev/sda1 /dev/sdb1
       
       Note: you may use more than two devices in a BTRFS RAID-1, and the devices do
             not necessarily have to be of equal size. For instance you could have a
             RAID-1 with 3 partitions of 100G, 100G and 200G. In such situation BTRFS
             will provide you with a RAID-1 of 200G. This works because every block of
             data is always copied to two devices, even if there is more disks.
       
       Add a new device to an already existing RAID:
       # btrfs device add /dev/sdb1 /srv
       
       Remove a device from the RAID:
       # btrfs device remove /dev/sda1 /srv
       
       See the devices that are part of the RAID:
       # btrfs fi show /srv
       
       Convert an existing BTRFS filesystem to RAID-1 by adding a second device:
       # btrfs device add /dev/sdb1 /srv
       # btrfs balance start -dconvert=raid1 -mconvert=raid1 /srv
       
       See the status of an ongoing RAID conversion:
       # btrfs balance status /srv
       
       Perform an extensive sanity check of the data (validates data checksums):
       # btrfs scrub start /srv
       
       See I/O error counters for devices in the RAID:
       # btrfs device stats /srv
       
       
       ! IMPORTANT NOTE !
       
       By default, Linux will fail to mount a BTRFS filesystem if any of the devices
       is missing. This means that should one of your RAID disks fail, your system
       will become unbootable.
       
       To make Linux mount a BTRFS RAID-1 filesystem even if one of its devices is
       missing, you have to pass the 'degraded' mount option. Here an fstab example:
       
       UUID=588b054f-c57e-4d6e-936a-4e67e6d2a075  /srv  btrfs  defaults,degraded  0 0