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Paper review 3 : Xen and the art of virtualization

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Paper review 3 : Xen and the art of virtualization

Summary & Goal:

This paper introduces Xen, a Virtual Machine Monitor that allows numerous operating systems to coexist simultaneously in the same hardware machine while maintaining high performance. Additionally, the paper discusses the virtualization approaches, design, implementation and performance evaluation of Xen hypervisor which focuses on providing virtual machine isolation for the multiple operating systems.

The goal of this research is to allow up to 100 virtual machines to run in the same hardware machine. These machines have to be isolated and must be able to support different multi-application OS with little overhead.

The positive contributions of the researched approach:

The author inputs much details on the benefits and architecture of the VVM which helps the users understand the context of XEN.

The paper introduces the notion of partial virtualization (paravirtualization) where the virtual machine abstraction is similar but not identical to the underlying hardware. This helps avoiding the costly performance penalties, such as efficiently virtualizing the x86 MMU, that arise from providing full virtualization to the x86 architecture. Besides, this has the added benefit of providing the operating system with real as well as virtual resources.

In order to minimize the effect of virtualization on OS sub-systems, Xen uses Borrowed Virtual Time Scheduling Algorithm that has a special mechanism for waking up a domain when it receives an event.

For the issue of performance isolation that is provided by traditional OS techniques which affect the performance of concurrent processes and thus, undesirable for a multi-programming system, The XEN framework suggests OS level multiplexing instead of process level multiplexing as the solution to this problem.

The VMM is designed such that instead of multiplexing across processes, it instantiates an entire OS for each request. This enables each user to run complex configurations between its processes and also ensures isolation between different OS’s.

Weaknesses & Reflections:

The number of different operating systems that are supported by Xen is limited by the ones they have created modifications for. Xen is targeted for x86 architecture and it does not support hardware portability.

Only the Xenolinux has been tested. It is not proven that existing operating systems will be functional with its virtual memory monitoring.

Data sharing between different domains is not secured and controlled, eventhough some future work on that aspect is stated (universal buffer caches).

Each OS is expected to allocate buffers and pass to Xen before even accepting the network packets. This makes the OS susceptible to DOS attacks unless it provides really powerful network filter rules.

Improvements Suggested:

-There should be a testing for other different type of operating systems co-working in the virtual space.

-A simulation of multi-OS concurrency with XEN is needed for proof of concept.

- An explanation on how to


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