Ask HN: What is the current state of the art in time series storage?

I'm researching viable (i.e. modern) alternatives to RRDTool, but it seems like it's never really been surpassed. There's Whisper (part of the Graphite project) which is still a disk orientated format. I'm trying to avoid tools such as OpenTSDB which are pitched at people with dedicated monitoring infrastructure. I'm sure for most scales something like a disk-backed (read: correctly configured) Redis instance would be sufficient, but there's no well documented prior art in this area as far as I can see?