btrfs-replace man page
btrfs-replace — replace devices managed by btrfs with other device.
btrfs replace <subcommand> <args>
btrfs replace is used to replace btrfs managed devices with other device.
Cancel a running device replace operation.
start [-Bfr] <srcdev>|<devid> <targetdev> <path>
Replace device of a btrfs filesystem.
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 is 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
only read from <srcdev> if no other zero-defect mirror exists. (enable this if your drive has lots of read errors, the access would be very slow)
force using and overwriting <targetdev> even if it looks like containing a valid btrfs filesystem.
A valid filesystem is assumed if a btrfs superblock is found which contains a correct checksum. Devices which are currently mounted are never allowed to be used as the <targetdev>.
no background replace.
status [-1] <mount_point>
Print status and progress information of a running device replace operation.
print once instead of print continuously until the replace operation finishes (or is cancelled)
btrfs replace returns a zero exit status if it succeeds. Non zero is returned in case of failure.
btrfs is part of btrfs-progs. Please refer to the btrfs wiki http://btrfs.wiki.kernel.org for further details.