no reason, it's doable with a bit more cleanup. The I/O bits are encapsulated entirely in extstore.c with a relatively clean interface. it's not a layered setup

no reason, it's doable with a bit more cleanup. The I/O bits are encapsulated entirely in extstore.c with a relatively clean interface. it's not a layered setup, it's actually bolted all into the same hash table, which makes pluggable I/O systems not make much sense. it ensures that a lot of operations can't touch secondary (like miss, touch, delete, sets of new items, etc), which reduces load on the IO by quite a lot. edit: Also extstore itself will support NVM + SSD sort-of-layers soon enough. I'll be retesting that on the same optane+ssd machine in a couple weeks.