Not sure how many will need to implement their own storage engine, but knowing there’re a couple of projects that support pluggable engines (Project Voldemort, Riak) it might be that for special scenarios special engines could perform better. Now you can learn how to do it for Membase:
Right now we’ve got an engine capable of running get and set load, but it is doing synchronous filesystem IO. We can’t serve our client faster than we can read the item from disk, but we might serve other connections while we’re reading the item off disk.
Original title and link: Memcached/Membase: Writing Your Own Storage Engine (NoSQL databases © myNoSQL)