Do you just want a fast DB or do you want guaranteed response times
(usually needed for real time systems).  Note: I doubt that there is a
single DB that can give you guaranteed response times since they depend
largely on the amount of data and its organization.

Regards

   robert

Please take a look at eXtremeDB.

The transaction logging version journals transactions to a file for
persistence.

eXtremeDB is an in-memory database (written specifically for embedded
systems).

eXtremeDB supports conventional primary/foreign key relations, as well as
object identifiers and references.

The High Availability version satisfies the requirement for redundancy.

In-memory means fast (really fast) searches.

W.r.t. the other poster's comments about guaranteed response times:  He is
correct.  The best a real-time database can do is guarantee return of
control after a given amount of time.  If the operation can't be completed
in the time allowed, it will be cancelled.  (eXtremeDB supports this concept
through its time-cognizant two-phase commit protocol in the High
Availability version, which ensures that master and replica(s) won't wait on
each other longer than a prescribed amount of time.)  All that said, it is
possible to calculate the worst-case response time.  eXtremeDB will be more
predictable than most other databases (maybe all?) because there is no SQL
to parse/optimize/execute, no inter-process communication (except for High
Availability, but then the time-cognizant two-phase commit guarantees
response time), no disk or file I/O (except the Transaction Logging
version), and no calls to the C run-time.

Regards,
McObject LLC