start [options] <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>
Given the following filesystem mounted at /mnt/my-vault
Label: 'MyVault' uuid: ae20903e-b72d-49ba-b944-901fc6d888a1 Total devices 2 FS bytes used 1TiB devid 1 size 1TiB used 500.00GiB path /dev/sda devid 2 size 1TiB used 500.00GiB path /dev/sdb
In order to replace /dev/sda (devid 1) with a bigger drive located at /dev/sdc you would run the following:
btrfs replace start 1 /dev/sdc /mnt/my-vault/
You can monitor progress via:
btrfs replace status /mnt/my-vault/
After the replacement is complete, as per the docs at btrfs-filesystem(8) in order to use the entire storage space of the new drive you need to run:
btrfs filesystem resize 1:max /mnt/my-vault/