You can replace devices on a live file system without unmounting the file system or stopping any tasks that are using the file system. If the system crashes or loses power while the replacement is taking place, the operation resumes when the system next mounts the file system.

To replace a device on a mounted Btrfs file system, use the following command:

sudo btrfs replace start source_dev target_dev [-r] mountpoint

In the previous command, source_dev and target_dev specify the source device to be replaced (source device) and the replacement device (target device). The mountpoint specifies the file system that is using the source device. The target device must be the same size or larger than the source device. If the source device is no longer available, or you specify the -r option, the data is reconstructed by using redundant data that is obtained from other devices,. such as another available mirror. The source device is removed from the file system when the operation is complete.

Use the btrfs replace status mountpoint command to check the progress of the replacement operation and the btrfs replace cancel mountpoint command to cancel the operation.