Is there any reason why memcahced/redis doesn't provide external store as an interface and let people implement the load/store operations? I would like to be able to implement hierarchy of "hot(RAM) -> warm(NVM) -> cold(SSD) -> DB" style cache.
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.