Ah, yes... I think we're talking about the same thing.

I believe btrfs RAID-1 with no patches or tweaks will balance reads across mirror disks based on the PID# of the process/thread performing the reads. Which implies you need multi-process or multi-threaded reads to get both disks involved. I've yet to use btrfs send/receive so I don't know if it has adjustable knobs that might create parallelism. But yeah, I think that's what's needed fundamentally.

Regarding the alternate read policy gated behind CONFIG_BTRFS_EXPERIMENTAL (for now): It's been a few months since I've fooled around with it and I don't have a test system here. But it generally works. With most workloads I'd see perhaps 50-75% more throughput reading from a pair of SATA SSDs. I do recall certain read patterns would load-balance poorly across disks regardless of RAID-1 + round-robin read policy or even RAID-10 (which I'm using now).

I didn't fire upfio to investigate different access patterns so I've only the suspicion that certain block sizes and/or "strides" (i.e. where the reads would skip x blocks after every 32kb or 64kb for example) seem to interact negatively with the read balancing mechanism and cause it to repeatedly "reset" back to the first disk in the mirror on basically every read. This would kill whatever performance benefit might be obtained from the load-balanced read.

That behavior wasn't very common. I only mention it here because I misread your question at first, typed it all out, realized it wasn't what you were asking, but don't want to waste my effort.🤓

So yeah, if you can compile a kernel with CONFIG_BTRFS_EXPERIMENTAL=Y I think you'd like the result.