Dumping Lots of Data to Disk in Realtime?
This family of databases is the heart of sendmail, and some SQL engines are built on top (MySQL if memory serves).
The interface is a model of simplicity: pointers to arbitrary length buffers for keys and data. All you need is key scheme that provides the post acquisition access that you require.
Berkeley offers hash and BTree style organization of the keys.
It may use memory mapped FileIO under the hood and handles all transfer of multiple buffers.
It provides multipe files or multiple tables in one file and you can control the cachesize.
It can run 2,000 inserts per second on hardware from the mid 90s. (UltraSparc II 450)
Berkeley DB (www.sleepycat.com)
As far a I know it runs on just about everything including several embedded OS's, Windows and every variant of Unix.