The fully-open Xen platform with a genuinely excellent web UI: XCP-ng as the community successor to Citrix Hypervisor/XenServer, Xen Orchestra for pools, live migration, backup, replication, and a clean REST API. The type-1 Xen architecture, the XO-from-source versus paid-XOA friction, XOSTOR hyperconverged storage, the pool-level-not-DRS placement gap, and where it fits versus Proxmox and the KVM crowd.
Xen
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XCP-ng and Xen Orchestra: The XenServer Successor