Database setup for datalogging
We are doing something very similar at my place and this very question was in my head for a while a few weeks back.

After noticing that logs were consuming 98% of our DBs in SQL Server I did some research on how we should handle this problem.

It all depends on the structure of your logs but our logs are not really relational so storing them in a RDBMS and consuming all the resources built for the RDBMS does not make sense.

I started looking into NoSQL stores and found DB Engines Ranking to be very useful to compare the feature sets of each DB engine.

Personally I found that an eventual consistency solution worked for me as it has good sharding, availability, and fast reads on huge data sets.

Check the CAP Theorem wiki entry to learn about the trade offs on different types of NoSQL solution.

We are currently testing a Cassandra deployment for our logs with MapReduce jobs.