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Ganeti
Ganeti is a virtual machine cluster management tool built on top of existing virtualization technologies such as Xen or KVM and other open source software.
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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.
Maybe not "simple", but Ganeti is pretty powerful. A Ganeti + Ceph combo will build you a hyperconverged system. Ganeti will scale to hundreds to thousands of servers. Plus it can build multi-cluster setups, where you can move VMs between clusters easily.
NOTE:
The number of mentions on this list indicates mentions on common posts plus user suggested alternatives.
Hence, a higher number means a more popular project.