Can't you just pass in a raw block device, get rid of any fsyncs and you got some very volatile backing store? The OS will write things out as best as it can, and if the whole thing fits in memory (and it's a memcached instance, so it should after all), then read-write will be fast. And if you want to restart, just stop the memcached process, do a sync, wait for it ... and reboot. (This seems simpler than copying the tmpfs, but a lot less safe/deterministic?)
You can try, but it's not going to be very consistent. You're at the mercy of whenever the OS decides to start flushing pages.. head of line blocking, mmap_sem locks, file descriptor locks, etc. My old job had an mmap-on-disk storage engine at scale and it wasn't any fun.
I think the worst part is mmap-on-disk looks fine at first, and only comes out as a problem after you scale up a while. False sense of security. :/
