Virtualization - Some General Concepts

7/31/2019 Virtualization - Some General Concepts

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Virtualization Technology

Prof D M Dhamdhere

CSE Department

IIT Bombay

Moving towards Virtualization Department of Computer Science and Engineering, IIT Bombay


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Some Benefits of Virtualization

Server consolidation

Also called workload consolidation

Better security and reliability forapplications using same host

Testing of software concurrently with

production use of a host

Disaster management



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What is Virtualization?


To understand virtualization, let us begin

with a virtual resource


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Virtual Resources


Definition

Illusion of a resource, supported by an OSthrough use of a real resource

Motivation To have more number of resources available

To have more of a resource available, e.g.,virtual memory

Historical

The concept originated in 1960s!


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Virtual Resources

A virtual resource is an abstraction

The OS implements the abstraction

through use of real resources

The implementation can be changed

without affecting the application using the

virtual resource

An application can be migrated to any host

offering the same abstractionportability



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Virtual Resources

Use of virtual resources started with I/O

devices

Motivation: Have more I/O devices available

Each user could have own I/O device

Then came virtual memory

Motivation: Have larger memory

And then .. Virtual machines

Motivation?



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Virtual Machines

Motivation: Provide a machine to each

user so that a user could use an OS of

own choice



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Virtual Machines

A virtual machine could be used in other

contexts as well

Motivation?

Since it is an abstraction, it can be made to have

desirable properties, e.g., simplicity

Some programming languages have their own

virtual machines, e.g., Pascal, Java

If implemented on several hosts, it provides

portability



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What is Virtualization?

Definition:

Virtualizationis the process of mapping the

resources and interfaces of a virtual resource

into the resources and interfaces of a host

machine.

Issues in virtualization

Complexity Correctnessincludes reliability, security

Performance! (overhead of the mapping)



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Overview of Virtualization

Virtual device virtualization

Map an operation on a virtual resource into anoperation of a real resource, e.g. virtual disks

Could be achieved through minimal overhead OS may map several virtual devices into same real

device

Virtual memory virtualization

Virtual memory manager maps parts of thevirtual memory into real memory

Overhead: loading and removal of pages



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Virtual machine virtualization

Application running under a virtual machineexecutes an instruction of the virtual machine

(we call it the guest machine) The actual operation should be implemented

through execution of instructions of the realmachine (we call it the host machine)

It should incur minimal overhead Hence it should be achieved through minimal

interpretation



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Who performs virtualization of virtual

machines?

A virtual machine operating system(VM OS)

A virtual machine monitor (VMM)

Also called a hypervisor

Not a full-fledged operating system



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VM/370

Provides virtual machines that have a subsetof the capabilities of the real machine

Implements a virtual machine throughvirtualization

Low overhead: Does not interpret each instruction

Lets an applications code execute on the real

machine most of the time Multiplexes various virtual machines

Overlaps their operation so that each virtualmachine receives a fair opportunity to operate



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VM/370

Schedules a virtual machine

Makes arrangement to regain control at

critical times

Issue: Ensure non-interference between

virtual machines



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VM/370

Ensures non-interference

Does not allow a virtual machines OS (i.e, guest

OS) to execute privileged instructions Runs the guest OS in user mode of CPU

Drawback: loses the distinction between a

guest OS and an application running under a

guest OS



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VM/370

How does it control overhead?

Allows a virtual machine to use instructions of the

CPU directly during its operation

No overhead at such times

OS of the guest machine is in control

Interrupts occurring during guest OS operation can

be handled directly by the guest OS



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Performing Virtualization

Full virtualization

Host machine and guest machine have

identical capabilities

Ad hoc arrangements may be need to ensure non-interference

Paravirtualization

Some host machine instructions are notvirtualized; their use is replaced by use of

other instructions



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Performing Virtualization

Paravirtualization (contd)

Achieved in two ways

Port a guest OS to work under a VM OS or VMM

Replaces use of nonvirtualized instructions byother instructions while installing a guest OS

Perform dynamic binary translation

Perform substitutions of nonvirtualized

instructions during operation

Allows a guest OS to be used readily



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Supporting Virtualization

Hardware features

Provide more CPU modes

Prevents interference by letting host OS, guest OS

and application use different CPU modes

Provide easily virtualizable features



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Supporting Virtualization

Software support

Ensure noninterference

Provide performance isolation

Reduce overhead

Replace control transitions of the kind application

host OS guest OS by application host

OS

Replace data transfers application guest OS

host OS device by application .. device



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Benefits of Virtualization Revisited

Workload consolidation

Create several virtual machines on a host

Security and reliability of applications

Create a virtual machine for each application

Test an application concurrently withproduction use of a host

Use separate virtual machines Disaster management

Port a virtual machine



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References

These slides are based primarily on Chapter 4 of

the book:

Operating SystemsA Concept-Based Approach,

D. M. Dhamdhere, McGraw-Hill, 2008

Mo ing to ards Virt alization D f C S i d E i i IIT B b

Virtualization - Some General Concepts

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