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156 lines
6.3 KiB
Markdown
156 lines
6.3 KiB
Markdown
---
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title: Lexicon
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description:
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published: true
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date: 2022-01-12T15:43:12.102Z
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tags:
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editor: markdown
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dateCreated: 2021-11-13T11:58:43.776Z
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---
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# Key terms related to virtualization
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## Emulator
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Emulators or virtualizers are software that provide material components
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similar to physical hardware, but that are made of computer code instead
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of silicon, -- virtual hardware --, such as virtual floppy disks.
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QEMU is a popular emulator that can act as a simulator or virtual
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machine monitor. In the latter case, it can leverage hardware
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acceleration,
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## Hardware-assisted virtualization
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Hardware-assisted virtualization is a feature of certain computer
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hardware made to take advantage of virtualization.
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Such hardware reduces the overheads associated with virtualization and
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is thus key to unlocking near-native performance for virtual machines.
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In other words, hardware-assisted virtualization translates into better
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performance for virtualized workloads, significantly reducing the gap in
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performance between a virtual machine and a physical one.
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## Nested-virtualization
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Nested-virtualization refers to the ability to run a virtual machine
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inside another virtual machine.
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## Virtualization
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Most computers are made of hardware and software. By analogy, the brain
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that animates the cells to control a body can be thought as the
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operating system that controls components of its body.
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Virtualization can be defined as the ability to run a software-based
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computer inside a physical computer. , It is a set of computer-related
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techniques that make it possible to create replicas of computer hardware
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out of computer code. Those replicas are often referred to as virtual
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machines.
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There are roughly three types of virtualization:
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* **Simulation or emulation**: when a computer is fully emulated and
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can be made to look like any device to an operating system
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* **Partition**: when computer resources are split such that each
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operating system can only see a subset of available hardware
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resources
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* **Paravirtualization**: when both hardware and software-assisted
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virtualization is being used. In this case, the guest is aware that
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it is running in the virtualized environment, and acts accordingly.
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Virtualization is used to better isolate resources on a physical
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computer and to distribute them across various workloads, enabling
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better use of resources through consolidation. For instance, with
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virtualization, multiple operating systems can run concurrently on a
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physical machine.
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## Hypervisor
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A hypervisor is an operating system or firmware that is designed to run
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guest systems: it handles scheduling, execution of hyper privileged
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instructions, memory management and over-commitment, and provides
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drivers for physical devices.
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The Virtual Machine Monitor (VMM) is a software that runs on top of the
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hypervisor and manages the life cycle of a virtual machine. It provides
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device models for emulated devices and implements tasks such as start,
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suspend, migrate, and stop virtual machines.
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The hypervisor and VMM work in tandem with emulators, which provide them
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virtual hardware.
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As of 2021, there are two major open-source hypervisor that are both are
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able to leverage hardware-assisted virtualization:
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- Xen (2003)
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- Kernel-based Virtual Machine (KVM) module for Linux (2007)
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## Device
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Devices are computer components that can be attached to machines. They
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can be classified in two ways : physical or emulated.
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| | Physical hardware | Emulated: model-based | Emulated: paravirtual |
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| :- | :-: | :-: | :-: |
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| *Design* | Specific | Specific | Generic |
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| *Type* | Silicon-based | Software-based | Software-based |
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* Physical
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* Physical components refer to devices that can be attached to a
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system. For instance, a dedicated physical graphics card
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attached to a physical system can be directly attached to a
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virtual machine, which then becomes responsible for managing it,
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a technique called *passthrough*. The PCI-SIG standards provide
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IOMMU-related specifications to allow a host operating system to
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not have a driver for a particular device and passthrough
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the device to the guest. The guest will have a device with
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nearly native performance, and use the standard vendor's drivers
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for the device.
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* The PCI-SIG standards also provide a way to partition compatible
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devices using so-called Virtual Functions (VFs). In this case,
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the host manages the way a physical device is used by the guest.
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Both host and guest must have specific device drivers. It offers
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nearly native performance.
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* Emulated
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* Model-based
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* Model-based emulated hardware are designed after real
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devices, but are made out of computer code, not silicon. The
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i440fx and Q35 chipsets are both instances of emulated
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hardware. This is the slowest (but most compatible) way to
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provide a device to a guest. An emulated GPU is not going to
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be fast enough in an emulated mode to do 3D rendering.
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* Paravirtual
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* Paravirtual hardware, also known as paravirtualized Virtual
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I/O devices or simply virtio, are also made out of computer
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code. Contrary to emulated hardware, they function as a
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generic piece of software-based hardware which doesn't
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replicate a specific hardware component.
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### Paravirtualization
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Paravirtualization refers to the emulation practice of letting an
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operating system running in a virtualized environment know that it is
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running in such an environment.
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Under this configuration, more efficient communication methods are
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available between the host and the guest, including VirtIO devices
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(e.g.: virtio-net for network devices, virtio-blk for block storage
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devices). Such devices can communicate directly with the host, instead
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of emulating every single command of an IDE, SATA, SCSI or NVMe device,
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as it is the case for model-based emulation.
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### Virtual machine
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A virtual machine is a recreation of a real, physical, silicon-based
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computer using software. It performs almost exactly as a physical
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computer, and can thus host an operating system.
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The expression "virtual machine" is often abbreviated VM. VMs are also
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often referred to as guests, in contrast to the hosts that host them. |