Hypervisors
Type 1 and type 2 hypervisors, their role in sharing processor, memory and input-output, the virtual CPU, overcommitment, memory ballooning and live migration.
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- 6 topics
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Hypervisor: what is it?
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Software that creates and runs virtual machines by dividing the physical machine's processor, memory and input-output among them.
It also isolates the machines from one another: a VM can read neither another VM's memory nor the host's.
A host with 32 physical cores carries VMs totalling 96 vCPUs, and everything works. How is that possible?
- The vCPUs do not all compute at once: the hypervisor runs them on the cores in turn
- Each vCPU is a third of a core that the hypervisor permanently reserves for its VM
- Memory ballooning takes back from the guests the cores they are not using
- The hypervisor emulates in software the 64 vCPUs that have no physical core
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The vCPUs do not all compute at once: the hypervisor runs them on the cores in turn
This is CPU overcommitment, here 3 to 1: a vCPU is not a reserved slice of a core, the hypervisor schedules it on a free core when the VM has work. It holds as long as the VMs do not all compute at once; otherwise CPU ready climbs.
A type 2 hypervisor is installed on top of a …; a type 1 hypervisor is installed directly on the ….
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A type 2 hypervisor is installed on top of a host system; a type 1 hypervisor is installed directly on the hardware.
The difference in type says nothing about the tool's power: it only tells what lies under the hypervisor.
Type 1 hypervisor: what is it?
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A hypervisor that runs directly on the hardware, with no general-purpose operating system underneath.
ESXi, Hyper-V, Xen and KVM are examples. There is one layer less than with type 2, so better performance: it is the usual choice for production servers.