Before mounting a multiple device filesystem, the kernel module
must know the association of the block devices that are attached
to the filesystem UUID.
There is typically no action needed from the user. On a system
that utilizes a udev-like daemon, any new block device is
automatically registered. The rules call btrfs device scan.
The same command can be used to trigger the device scanning if
the btrfs kernel module is reloaded (naturally all previous
information about the device registration is lost).
Another possibility is to use the mount options device to specify
the list of devices to scan at the time of mount.
# mount -o device=/dev/sdb,device=/dev/sdc /dev/sda /mnt
Note
that this means only scanning, if the devices do not exist in
the system, mount will fail anyway. This can happen on
systems without initramfs/initrd and root partition created
with RAID1/10/5/6 profiles. The mount action can happen
before all block devices are discovered. The waiting is
usually done on the initramfs/initrd systems.
RAID5/6 has known problems and should not be used in production.
