Explain that virtualization is a technology that lets physical resources be used as if arranged differently from how they really are, and that server virtualization (サーバの仮想化) splits one physical machine into several independent environments.
Tell apart the three main approaches to server virtualization by what sits on top of the hardware and whether each virtual machine has its own OS.
Explain that VM and virtual machine mean the same thing, and that it runs by sharing the resources of a physical machine.
Virtualization (仮想化) is a technology that handles physical resources such as the CPU, memory, and disks in software, so they can be used in a configuration different from the real one.
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A typical use is server virtualization, which divides one physical computer so it can serve as several environments. When each divided environment is built as a virtual computer, each one is called a virtual machine, and the OS running inside it is the guest OS. The reverse, bundling several physical resources so they appear as one, is also virtualization.
AnalogyThink of dividing a large open floor with partitions and renting it out as many private rooms. There is only one building, but each tenant uses their room as their own.
How to rememberVirtual means "not the real thing, but working as if it were." The same word gives the virtual in virtual machine and virtual memory.
Exam points
ExamServer virtualization (サーバの仮想化) logically divides one physical machine so that several servers can run on it at the same time.
ExamIn server virtualization, the physical machine's resources can be allotted to each divided environment in the amounts it needs, so resources can be shifted flexibly to match demand.
Mix-upVirtual memory (仮想記憶) is a mechanism that uses auxiliary storage to make main memory look larger than it really is, while server virtualization is a technology that uses one computer as several independent computers.
The hosted type (ホスト型) installs virtualization software on the OS you normally use (the host OS) and runs virtual machines on top of it.
Hardware
Host OS
Virtualization software
Guest OS
Applications
You can set it up much like adding one more application to an existing PC, so it is easy to try out. On the other hand, whenever a virtual machine uses the hardware, the work passes through the host OS, and that extra load (overhead) tends to slow processing down.
AnalogyIt is like a homeowner renting one room of the house they live in to a guest. The guest can use the room freely, but must go through the owner's home to reach the front door, kitchen, and bathroom.
How to rememberA host is the one who receives guests, and a guest is the visitor. The guest OS sits on top of the OS playing the host.
Exam points
Mix-upThe host OS (ホストOS) runs directly on the physical machine, and the guest OS (ゲストOS) runs inside a virtual machine (仮想マシン); they are told apart by where they run.
The hypervisor type (ハイパーバイザー型) places control software dedicated to virtualization (the hypervisor) directly on the hardware and runs virtual machines on top of it.
Hardware
Hypervisor
Guest OS
Applications
The hypervisor is in charge of allotting CPU and memory to each virtual machine. Few layers sit between the guest OS and the hardware, so this type is widely used on servers that run many virtual machines together.
AnalogyIt is like building an apartment block with self-contained units straight onto an empty lot. With no owner's house in between, residents of each unit can come and go easily.
How to rememberThe core of an OS was once also called the supervisor. Standing one level above it (hyper) and overseeing several OSs together is the hypervisor.
Exam points
Mix-upHypervisor (ハイパーバイザー) is the name of the software that controls virtualization (仮想化), while hypervisor type (ハイパーバイザー型) is the name of the approach that places it directly on the hardware.
Hosted (Host OS) type vs. Hypervisor type
Aspect
Hosted (Host OS) type
Hypervisor type
Key differenceDirectly on the hardware
Host OS
Hypervisor
Layers between hardware and guest OS
Host OS and virtualization software
Hypervisor only
Processing overhead
Tends to be larger, since work passes through the host OS
The container type (コンテナ型) shares the core of one host OS (the kernel) while separating only the application runtime environments (containers).
Hardware
Host OS (kernel)
Containers (apps and components)
A container has no guest OS; it bundles only the application and the components it needs to run. Because no full OS has to boot, it starts quickly and uses less memory and disk space. In exchange, every container relies on the host OS kernel, so it is not suited to running an OS with a different kernel inside.
AnalogyIt is like a shared office where all tenants share the reception desk and utilities, and each company has only its own partitioned booth.
How to rememberA shipping container moves its contents from carrier to carrier as one box. A software container likewise packs an application and the parts it needs into one box to carry around.
VM is an abbreviation formed from the initials of Virtual Machine, and it means the same thing as a virtual machine (仮想マシン).
Product names, admin screens, and technical documents often shorten virtual machine to VM. To distinguish it, the real computer is called the physical machine.
Exam points
Mix-upVM is not the name of the technology of virtualization; it refers to each individual virtual computer created through virtualization.
A virtual machine (仮想マシン) is a virtual computer, realized in software, that virtualization creates on top of a physical machine.
Each virtual machine is given part of the physical machine's resources, and its guest OS and applications run within that share. Virtual machines are independent of one another, so a failure in one is unlikely to spread to the others. Since a virtual machine is really just files and data, it is also easy to copy or move to another physical machine.
AnalogyIt is like the units in one apartment building. There is only one building, but different people live in each unit, and noise next door rarely reaches yours.
Exam points
ExamEach virtual machine (仮想マシン) can have its own separate OS installed and run as an independent computer.
Virtualization presents physical resources in a configuration different from the real one, and with server virtualization one physical machine can serve as several independent environments.
The hosted type installs virtualization software on an everyday OS and is easy to add to an existing PC.
The hypervisor type places control software directly on the hardware and is widely used on servers that run many virtual machines.
The container type shares the kernel and separates only the application runtime environments, so it consumes few resources.
A virtual machine (VM for short) is a virtual computer with a guest OS that runs by sharing the physical machine's resources.