CouchDB is very reliable, provides excellent durability, and you'll experience very low CPU load. It's also excellent at replicating between multiple nodes, either on-demand or continuously.
Thanks to its replication abilities and RESTful API (it uses HTTP for its API) you can scale horizontally pretty easily using mature tools. (Nginx or Apache for reverse proxying, HTTP load balancers, etc.)
You write map/reduce functions in JavaScript to precompute queries. The results are built up incrementally on disk which means they only neeed to be computed once per signal. In other words, queries can be really fast because it only has to do calculations on the signal data recorded since the last time you ran the query.
CouchDB trades disk space for performance, so you can expect to use a lot of disk space. Your queries can be lightning fast and conserve disk space if you implement them properly.
Give CouchDB a try.
Check out Why Large Hadron Collider Scientists are Using CouchDB and CouchDB at the BBC as a fault tolerant, scalable, multi-data center key-value store
