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InfluxDB
Power Real-Time Data Analytics at Scale. Get real-time insights from all types of time series data with InfluxDB. Ingest, query, and analyze billions of data points in real-time with unbounded cardinality.
Simplicity of file format is also a factor (e.g. Ennui has its world in very neat yaml files), but not that important.
Looks like its site is down, but there's a copy of it here. It looks promising, thank you!
NOTE:
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