Flush will be able to flush into multiple files under this design, avoiding L0 I/O amplification.
I have the patch which is missing just one feature - support for concurrent flushes and stripe changes. This can be done via extensive try-locking of stripe changes and flushes, or advisory flags without blocking flushes, dumping conflicting flushes into L0 in case of (very rare) collisions. For file loading for the latter, a set-cover-like problem needs to be solved to determine optimal stripes. That will also address Jimmy's concern of getting rid of metadata, btw. However currently I don't have time for that. I plan to attach the try-locking patch first, but this won't happen for a couple weeks probably and should not block main reviews. Hopefully this will be added on top of main reviews.