strfry uses lmdb, same stuff most crypto daemons use to store the blockchain. It's supposed to be quite good (haven't used it), especially since you aren't so much doing relational shit with it, just reliably storing data in blocks on disk. I believe it is what slapd uses btw, if you've ever used that. Iirc it was spawned from that project and grew into its own thing, so I don't reckon it sucks ass