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rclone
"rsync for cloud storage" - Google Drive, S3, Dropbox, Backblaze B2, One Drive, Swift, Hubic, Wasabi, Google Cloud Storage, Azure Blob, Azure Files, Yandex Files
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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.
I'm using the Bridge together with offlineimap and then have borgbackup taking backup of the data to enable point-in-time recovery. The restore step is a bit clunky, but this setup does works.
It is also possible to combine local storage with cloud services using rclone, which can add yet another encryption layer. This hybrid mode will also use the cloud storage more as a disaster recovery scenario and the best performance (as the ProtonMail backup process will only access the local storage).