OpenTSDB scales well, 10 million DPS writes on a 36 node cluster for example. It's not easy to get working well at scale out of the box. That is something we really need to work on in that community. There is a project called Splicer (github.com/turn/splicer) that will shard the incoming queries into 1 hour blocks and cache the 1 hour blocks, it also sends the queries to the same node where the region server is that hosts the data. This makes the queries VERY fast.