An Overview of Virtualization Technology

Jane Walker writes to tell us that TechTarget has a short writeup on virtualization and some of the ins and outs of using this technology effectively. From the article: "Virtualization is a hot topic in the enterprise space these days. It's being touted as the solution to every problem from server proliferation to CPU underutilization to application isolation. While the technology does indeed have many benefits, it's not without drawbacks."

Psst. btw

[BadAnalogyGuy]

Microsoft has made their server virtualization software available for free.

http://www.microsoft.com/windowsserversystem/virtu alserver/software/default.mspx [microsoft.com]

Windows Licence Issues. (wrt. Virtulization)

[Domini]

One thing the article does not speak about is licensing issues when using Virtulization. For instance MS has some twists and turns...

For instance:

One needs 2 different licenses if you run XP in XP.
You can run 4 instances of Windows Server for free in Windows Virtual Server.
You can run one copy of an older windows for free in Windows Vista.
(You can read more about this on the MS site...)

For Windows XP General Purpose license User Rights:

http://www.microsoftvolumelicensing.com/userights/ PUR.aspx [microsoftv...ensing.com]

Download and read document, section "Microsoft Desktop Operating Systems" which reads:

  I) Installation and Use Rights.
    a) You may install up to two copies of the software on one device.
    b) Except as provided in Section II.a and II.b below, only one user may use the software at a time.
    c) You may run a prior version in place of the licensed version for either or both of the copies.
    d) You may only use the copies on the device on which you first install them.
    e) You may use the software on up to two processors on that device at one time.

Thus this means that I can install and use XP as Bootcamp native and Parallels VM guest using only one license.

yay!

Re:Good reading until the end

[IO ERROR]

Looks like you took at least one of those quotes out of context. Here's the context:

If you're a developer looking for a flexible way to test your application in multiple environments, you'll probably want to go with either Virtual PC or VMware Workstation. If you want to use Linux as your host OS, you'll definitely have to go with VMware.

Virtual PC doesn't run on a Linux host, so you'll definitely have to go with VMware.

what a horrible article

[jnf]

Seriously, how did this make it on /.? The article is only a few paragraphs long, doesn't really even touch on hardware virtualization support or why its necessary (because virtualization currently sucks under 'normal' intel architecture). It even refers to qemu as virtualization, which its not, its an emulator. It mentions the program once then never touches on it again. It never explains why a person might want to use bochs or qemu even though its much slower than vmware/virtual pc. it doesn't touch on parallels [parallels.com] or any other software out there.

Even more it doesn't even explain why the suggestions it makes are made. This article is basically a badly written advertisement for vmware or virtual pc.

Uses of virtualization for servers

[mpcooke3]

I have been using a few Xen based virtual servers from a commercial company recently - I used to manage physical machines. Here are some of my thoughts:

Advantages:
* Low performance overhead of Xen compared to other virtual solutions, and full OS level access as if it was a normal server.
* The cost of a hosted Xen solution is very low given that the hardware is usually managed.
* Reduced/No trips to the data center to replace hard disks etc,
* From the provider i use you can also reinstall the OS, snapshot and restore snapshots over a web interface and get access to the console. These are features you can set up in your own data center but most people never get round to.
* Quicker turn around if you need new servers, since normally they already have the spare hardware it's 1 or 2 days to get a new server set up rather than 1 to 2 weeks to order, install and configure it.
* You could do loadbalancing over several Xen Virtual hosts on physically separate machines very cost effectively. This would also mitigate against the variable performance on different Xen hosts if you used a dynamic weighting loadbalancer.

Disadvantages:
* Sometimes other users on the Xen system cause problems, or the server is restarted due to Xen related problems. This hasn't happened that often but you wouldn't currently run a system that needed 99.999% availability on a XEN virtual host if the system is vulnerable to a single server going down.
* You never know quite what your worst case performance is going to be like.
* If your system doesn't scale laterally (more servers) but only by buying a more powerful single server (some databases for example) then the Xen virtual hosting is not cost efficient.

Re:Good reading until the end

[Torne]

You can't run Xen on Linux - Xen is a freestanding hypervisor that runs directly on the metal. So, the statement in the article seems perfectly reasonable. (this is, incidentally, one of the (numerous) advantages of Xen over VMWare in the performance stakes - being able to control the hardware directly instead of having to mess around with what the host OS will let you do is quicker).

Every OS on a Xen system is a guest OS. Some of them just have permission to create new OS instances, or access particular bits of real hardware directly.

Re:Good reading until the end

[Hercynium]

Xen is a freestanding hypervisor that runs directly on the metal.

I feel I should get a little pedantic with this statement. Xen, specifically, is a modification of the linux kernel that provides hypervisor capabilities for the host OS (Linux) and integration with the guest OSes. Xen's host components can't run directly on the metal, like VMWare ESX can. It needs the rest of the linux (I can't remember if it's been ported into other kernels) kernel to provide hardware access. Also, Xen requires (until pacifica, et al) that the guest OS kernels be modified to integrate with the host's hypervisor layer. Without that, Xen does nothing. (VMWare ESX does not require a modified guest OS)

So, if you think of a Xen-enabled linux kernel as Xen, you're right. But I see it as a seperately developed, ported, and integrated extension that requires a kernel to operate. Again - I believe there are efforts to get it running inside other kernels, but I don't remember.

On second thought - someone fill me in here - I'm guessing that VMWare ESX probably runs as part of some ther's OSes HAL, but of course they don't say so in the sales pitch...

*disclaimer* -- While I really like VMWare's product for functionality and ease-of-use, for performance I'd go with Xen. I'm currently involved with a project at my company to virtualize as much of our datacenter as possible, and I've been pitching Xen to the group, over VMWare, provided XenSource's product lives up to it's marketing specs.

Re:Good reading until the end

[tinkertim]

Correct. I work with Xen daily and most of my products and services are built around it. One of which is a replacement for Virtuozzo for the purposes of maximizing and isolating resources or web hosting companies.

Xen augments the kernel, it does not replace it. The Xen hypervisor then interacts with the host (dom-0) kernel.

dom-u (guest) images can then boot using any kernel modified to interact with the Xen hypervisor. Currently we play with:

Debian (Sarge)
FC4
CentOS 4
NetBSD

As dom-u's (guest) OS's.

We have also enjoyed some success but not 100% stability bringing Win2k3 up as a dom-u.

I have deployed clusters that use Xen as a management layer and I can tell you, it *does* live up to its marketing specs. Xen's bridging is the fastest most efficient layer available, bar none. Its also a wonderful tool in helping to integrate a centralized storage area network into any size network and let people keep all of the protocols they like.

A *very* good source of information about Xen, what it does, how it does it is available on the option-c wiki (Here [option-c.com]) , they also have some ready to go Debian installers that make installation quite easy (apt-get able).

Xen + OpenSSI is another fantastic combination if you take the time to really understand the networking possiblities and set it up appropriately. Good luck with the bean counters .. the price is right :)