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You can use InfluxDB or TimescaleDB.

A single raspberry pi?

Literally any database or disk hierarchy.

I'd probably use SQLite, or if I had any relational database up already, I'd use that.

For something that lightweight, LMDB could be the best.

I always posted such stuff to a carbon/whisper server, and viewed it with kibana.

...a CSV file?

seriously... simple n easy.

The raspberry pi is on spotty wifi and needs to sync the data to a data collection server now and then, but also have fairly low (5 min) latency.

CSV and rsync could do it.