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httm
Interactive, file-level Time Machine-like tool for ZFS/btrfs/nilfs2 (and even actual Time Machine backups!)
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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.
The only real advantage I can think of with nested ZFS is that the files in the KVM would obviously be individual inodes in the nested ZFS filesystem and datasets, in addition to the one inode per virtual volume on the hypervisor (or zvol). This would allow for granular file management on the kvm and the use of tools like https://github.com/kimono-koans/httm which is like a command line time machine for ZFS.
NOTE:
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Hence, a higher number means a more popular project.