Managing value pairs - the best approach?

Hi folks I am looking for the lowest overhead approach to managing persistent session data. Our session class will be doing at least one read and write to the to the session table for every page view, so it will be pretty resource intensive if we don't get it right. MySQL is weak in read/write scenarios, and running Postgres just for this seems over the top, so I am looking for a better approach. The data will simply be a value pair - Key = session ID, Value = serialised data array. The Sleepycat Berkeley DB would seem to be a good way to go. An alternative approach would simply be to use the file system, with a seperate file for each record. But I suspect that this might break down once records hit the tens of thousands. Or is there some other approach I am missing? Any advice would be much appreciated Geoff Caplan