Concluding Remarks Virtualization may bring several advantages to the de- sign of modern computer systems including better secu- rity, higher reliability and availability, reduced costs, better adaptability to workload variations, easier migration of vir- tual machines among physical machines, and easy coexis- tence of legacy applications. Many vendors including Sun, IBM, and Intel have already announced or already have virtualization solutions. Intel has just announced a new ar- chitecture, called Intel Virtualization Technology, that pro- vides hardware support for virtualization allowing the vir- tual machine monitor to run at a protection level below ring 0. Sun has introduced the concept of zones in Solaris 10, which allows for many Solaris 10 instances to coexist on the same machine (www.sun.com/bigadmin/content/zones/). IBM provides Logical Partitioning (LPAR) technology on its p, i, and z platforms. This technology was originally devel- oped for its mainframes but is now available on its midrange servers (www-03.ibm.com/servers/eserver/iseries/lpar/and www-03.ibm.com/servers/eserver/pseries/lpar/) It is important to briefly discuss two major directions for virtualization that can be encountered on the market. One is called full virtualization and the other paravirtual- ization. In the former case, the Virtual Machine Monitor provides an identical abstraction of the underlying hardware to the virtual machines. However, not all architectures are virtualizable [7]. The x86 architecture is such an exam- ple [9]. Paravirtualization can then be used to overcome these situations by providing an “almost” identical abstrac- tion of the underlying machine to the virtual machines. This abstraction implements some new virtual instructions so as to make the machine virtualizable. The drawback is that the guest operating system has to be modified to use these instructions while in the full virtualization case this is not required. Paravirtualization provides better per- formance than full virtualization since the guest operating systems are aware that they running on a VM and therefore can be optimized for that type of environment. Examples of virtual machine monitors that use paravirtualization in- clude the open source Xen [1] and Denali [12]. An example
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Figure 3: Server consolidation scenario.
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Figure 4: Performance comparison for server consolidation scenario.
of a virtual machine monitor that uses full virtualization is VMware (www.vmware.com). This paper presented some of the issues involved in us- ing well-known queuing network models for virtualized en- vironments. A case study on server consolidation was used to illustrate the point.