IMHO for all this class of problems, that is: need to log big amount of data over time for years, the way to go is not Riak, nor Redis, nor <put your preferred DB name here> but, simply writing to files in append only mode (and when you can, using a fixed size record for fast access later).
There are good reasons IMHO for writing a small networked C server doing this work.
I've recently discovered that when you know almost nothing about the problem someone is trying to solve it is both easy and attractive to speculate about how much better your solution is than the one reached by the people who have to solve it. Full time. For money. I see you have discovered the same. Great minds think alike?
There are good reasons IMHO for writing a small networked C server doing this work.