Download It

Virtualization Technology

Prof D M DhamdhereCSE Department

IIT Bombay

Moving towards Virtualization…

Department of Computer Science and Engineering, IIT Bombay

Some Benefits of Virtualization

• Server consolidation– Also called workload consolidation

• Better security and reliability for applications using same host

• Testing of software concurrently with production use of a host

• Disaster management



What is Virtualization?



• To understand virtualization, let us begin with a virtual resource

Virtual Resources



• Definition– Illusion of a resource, supported by an OS

through 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!

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 abstraction—portability



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?



Virtual Machines

• Motivation: Provide a machine to each user so that a user could use an OS of own choice



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



What is Virtualization?

• Definition:

Virtualization is 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• Correctness—includes reliability, security …• Performance! (overhead of the mapping)



Overview of Virtualization

• Virtual device virtualization– Map an operation on a virtual resource into an

operation 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 the

virtual memory into real memory• Overhead: loading and removal of pages




• Virtual machine virtualization– Application ‘running under’ a virtual machine

executes an instruction of the virtual machine (we call it the guest machine)

– The actual operation should be implemented through execution of instructions of the real machine (we call it the host machine)

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

interpretation




• 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




• VM/370– Provides virtual machines that have a subset

of the capabilities of the real machine– Implements a virtual machine through

virtualization• Low overhead: Does not interpret each instruction• Lets an application’s code execute on the real

machine most of the time

– Multiplexes various virtual machines• Overlaps their operation so that each virtual

machine receives a fair opportunity to operate




• VM/370– ‘Schedules’ a virtual machine– Makes arrangement to regain control at

critical times– Issue: Ensure non-interference between

virtual machines




• VM/370– Ensures non-interference

• Does not allow a virtual machine’s 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




• 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



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 not

virtualized; their use is replaced by use of other instructions



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 by

other 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



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



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’



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 with production use of a host– Use separate virtual machines

• Disaster management– Port a virtual machine



References

These slides are based primarily on Chapter 4 of

the book:

Operating Systems—A Concept-Based Approach,

D. M. Dhamdhere, McGraw-Hill, 2008

