Fastest and stable non-sql database? [closed]

Question

What is the fastest and most stable non-sql database to store big data and process thousands requests during the day (it's for traffic exchange service)? I've found Kdb+ and Berkeley DB. Are they good? Are there other options?

More details...

Each day server processes > 100K visits. For each visit I need to read corresponding stats from DB, write log to DB and update stats in DB, aka 3 operations with DB per visit. Traffic is continuously increasing. Thus DB engine should be fast. From one side DB will be managed by demon written on C, Erlang or any other low-level language. From another side DB will be managed by PHP scripts.

Answer 1

The file system itself is faster and more stable than almost anything else. It stores big data seamlessly and efficiently. The API is very simple.

You can store and retrieve from the file system very, very efficiently.

Since your question is a little thin on "requirements" it's hard to say much more.

Answer 2

What about Redis?

http://code.google.com/p/redis/

Haven't try it yet did read about it and it seem to be a fast and stable enough for data storage. It also provides you with a decent anti-single-point-failure solution, as far as I understand.