Btrfs RAID1 SSD + non-SSD

I asked on the btrfs mailing list, and got the answer I needed.

To summarize, the btrfs raid 1 disk access algorithms work by reading from one disk for even-numbered PIDs, and the other for odd-numbered PIDs. It's on the roadmap to upgrade this to a more advanced algorithm, but obviously that sort of lottery probably won't work out very well for one fast disk and one slow disk. And on top of that, it's possible that if the slower disk falls too far behind the faster disk while writing, it may cause errors.

I've opted to simply make backups to the slower disk and restore from those in the event of an error. That way I can still enjoy the bitrot protection of btrfs' checksumming while avoiding possible issues with different disk speeds.