start [-Bfr] <srcdev>|<devid> <targetdev> <path>
On a live filesystem, duplicate the data to the target device which is currently stored on the source device. If the source device is not available anymore, or if the -r option is set, the data is built only using the RAID redundancy mechanisms. After completion of the operation, the source device is removed from the filesystem. If the <srcdev> is a numerical value, it is assumed to be the device id of the filesystem which is mounted at <path>, otherwise it is the path to the source device. If the source device is disconnected, from the system, you have to use the devid parameter format. The <targetdev> needs to be same size or larger than the <srcdev>.
the filesystem has to be resized to fully take advantage of a larger target device; this can be achieved with btrfs filesystem resize <devid>:max /path
A valid filesystem is assumed if a btrfs superblock is found which contains a correct checksum. Devices that are currently mounted are never allowed to be used as the <targetdev>.
status [-1] <mount_point>