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Dave Elliman

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Xen and virtualization

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Virtual Machines

A virtualization layer is interposed between the hardware and the operating systems

Multiple operating systems can run on the same hardware simultaneously

They can be the same o/s or different Each is isolated from the others and unaware of their

existence A Virtual Machine Monitor is needed to accomplish this:

The VMM or Hypervisor The overhead must be reasonably small – this has

driven changes to chip design to support virtualization ( Intel VT (codenamed Vanderpool) and AMD's is referred to as 

AMD-V (codenamed Pacifica)

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The idea has caught on

An old idea in fact IBM 370 in 1972! Xen is far more flexible

Sun’s VirtualBox Vmware ESX Server Microsoft just released Hyper-V Xen is the most widely used by far –

available as open source but now owned by Citrix Inc.

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Used in web servers

You want to host a web site? You are likely to purchase a virtual server

It looks like it is your machine but really it is a virtual machine running under Xen

As a hosting company you can buy 50 or 100 times less hardware!

Not surprising this caught on

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Security can be Enhanced

Potentially huge security benefits Provides isolation between operating system instances

and multiplexes physical resources across the running virtual machines.

Security holes in the operating system can be caught in the hypervisor which has a simpler and leaner API.

It is easier to code a secure hypervisor than a secure O/S

Of course a vulnerability in the hypervisor could be a HUGE risk, spreading infection rapidly.

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Why only use 1 machine?

Hypervisors can cooperate across a cluster or farm of servers

Hypervisors can move a virtual environment from machine to machine

This can be done in say 200mS – the user does not notice a delay!

Why is this a brilliant idea?

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Scalability and Robustness

One can load balance the machines One can add more machines to the cluster

easily offering more performance Trees of clusters are possible for really

high performance A busy machine can offload some

processes to a less busy one An unreliable machine or one suffering

power failure can migrate its processes

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More of Xen

Developed by Ian Pratt at Cambridge

Bought by Citrix Systems in 2007

Hosts operating systems such as Linux and Windows. Some source code modifications are necessary especially for Windows

Is completely software based and requires no special hardware support.

Full virtualization, in which the virtual hardware is identical to the underlying physical hardware, is virtually impossible on the x86 architecture. New chips will solve this.

Xen provides a similar, but not quite identical view of the hardware.

Support unmodified application binaries.

Support fully functional, multi-application operating systems a guests.

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Porting an OS to Xen

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Requires less than 2% of the total lines of code to be modified.

The User Software The User Software runs on the Guest OS runs on the Guest OS without requiring without requiring modification.modification.

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Performance

Some overhead – 2% to 5% depending on memory and other resources

Xen seems to have a lower overhead than its competitors

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Enterprise Computing?

Highly relevant! Offers: Scalability Reliability Robustness Security

An arguably tidier solution than large enterprise application containers or special cluster operating systems

Xen + Spring + Hibernate and WCF with WPF Client looks pretty good to me.

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References

University of Cambridge Xen page http://www.cl.cam.ac.uk/Research/SRG/netos/xen/

Wikipedia entry for Xen http://en.wikipedia.org/wiki/Xen

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