Most commonly z/OS, z/VM as a hypervisor or just native Linux(rhel, sles, Ubuntu or Debian)
Depending on what model of mainframe you got you can use up to 60 logical partitions which are firmware level virtual machines. and can extend this functionality using z/VM or just plain KVM in a Linux environment
Depending on what model of mainframe you got you can use up to 60 logical partitions which are firmware level virtual machines. and can extend this functionality using z/VM or just plain KVM in a Linux environment