My laptop has two disks, a SSD and a traditional magnetic disk. I plan to make a partition on the mag disk equal in size the SSD and set up BTRFS RAID1. This I know how to do.
Although I am not routinely familiar with BTRFS, I do know today's file systems repair themselves, and when a mirror RAID1 used, the file system can operate only from one mirror if necessary. And with the right file system fsck.btrfs .. the repairs and read are performed from the first disk to access the information and then passed to the process which needs it.
The only reason I'm doing the RAID1 is for the self-healing. I realize writing large amounts of data will be slower that the SSD alone, but is it possible to set it up to only read from the magnetic drive if there's an error reading from the SSD?
In summary, to set it up to only read from the magnetic drive if there's an error will be done summarily as an auto process because of the RAID1 setup.