Comparing VM performance to physical servers

Performance and availability
Virtualization and hardware
How much performance loss is attributed to virtual machines (VMs) compared to stand alone servers? A VM server cannot be lossless because the VM software itself should consume some hardware resources.

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There is a small extra loss for the VM Server. However compared to the cost of the extra hardware needed to host the virtual machines, I’ve found that the loss is acceptable.

*** Improved by Wrobinson on 4/18/08 ***

The loss can be negligible or unberable depending on the configuration of the VM hosts, as well as the operational profile of the VM guests. There are different objectives for going virtual, some of which include consolidating assets, in which case, you will be interested in achieving the highest pack ratio possible. Alternatively, you may be looking to enhance business continuity and disaster recovery capabilities, in which case, the pack ratio may not matter as much. There is yet another possibility and that is archiving where storage will be the primary concern.

There is an old saying that there are several ways to skin a cat but I am a cat owner, so I am not interested in how many ways there are to do that. The saying is true, however, when it comes to virtualization. You need to come to identify your objectives which will in turn help determine the best design principles and practices to best achieve those objectives. If it is performance that you are after, then make sure to factor in processing, memory, fast disk drives and separate disk controllers.

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  • Amaltemara
    The overhead with virtualization depends greatly on the virtualization technology. Solutions that install onto the 'bare metal' without a Host OS will perform the best most of the time (VMware ESX being one), with solutions that install onto another OS taking much more of a hit. (VMware Server / Virtual PC). You have to really consider your environment and objective, and look for the solution that fits. There are virtualization platforms for web hosting that work just for Linux OSs, and can share the same OS filesystem, but run separate environments and kernels in memory. These solutions run even faster than ESX server, but only run 1 version of the OS. VirtualServer is free, but has to be installed on another OS, such as linux or windows. It has the disadvantage of much more overhead, but has the advantage of being able to use the hardware compatibility of the host OS. Virtualization solutions that run on the 'bare-metal' without a host OS are fast, but only support a select group of hardware, because they must write their own device drivers specifically for their solution.
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  • Anil Desai
    Unfortunately, there's no single number that can be used to demonstrate the amount of overhead for the many different types of guest OS's and applications you'll be deploying. In general, most organizations will find that the performance trade-off is minimal when compared to the cost savings and other benefits of moving to virtualization. If you really want numbers for planning purposes, you can try running standard CPU, memory, disk, and network benchmarks on a host server and within a VM on the host server. Of course, the challenge will be in translating this to "real world" impacts - something that often requires adequate performance monitoring and testing. In general, for most business applications, you're unlikely to run into insurmountable performance issues with VMs.
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