There are a few. I've used a modified LMDB source in the past with success, employing similar tricks to kdb for performance (i.e. store daily data as contiguous

There are a few. I've used a modified LMDB source in the past with success, employing similar tricks to kdb for performance (i.e. store daily data as contiguous arrays so that reads are quick etc). Either way, implementing a memory mapped store of arrays and operating on it is not too challenging a problem for any good C++ dev.