An Introduction to Full Virtualization With Xen (Linux.com)

Posted Oct 23, 2012 22:52 UTC (Tue) by cesarb (subscriber, #6266)
In reply to: An Introduction to Full Virtualization With Xen (Linux.com) by cyanit
Parent article: An Introduction to Full Virtualization With Xen (Linux.com)

> Why haven't they all switched to KVM already?

AFAIK, KVM needs HVM.

An Introduction to Full Virtualization With Xen (Linux.com)

Posted Oct 24, 2012 4:53 UTC (Wed) by stefanha (subscriber, #55072) [Link] (9 responses)

>> Why haven't they all switched to KVM already?
> AFAIK, KVM needs HVM.

No, KVM has always done HVM. HVM means using the hardware virtualization extensions (Intel VMX or AMD SVM). This allows unmodified guest operating systems to run.

Speaking of KVM, there is a project called Xenner to run Xen PV guests on KVM. More info here:

http://kraxel.fedorapeople.org/xenner/

[Disclaimer: I work on KVM]

An Introduction to Full Virtualization With Xen (Linux.com)

Posted Oct 24, 2012 7:35 UTC (Wed) by drago01 (subscriber, #50715) [Link] (8 responses)

> No, KVM has always done HVM. HVM means using the hardware virtualization extensions (Intel VMX or AMD SVM). This allows unmodified guest operating systems to run.

Your answer does not make sense, what he said is that KVM needs (as in requires) hardware virtulaization extensions to work, while Xen does not (for paravirualizied guests).

An Introduction to Full Virtualization With Xen (Linux.com)

Posted Oct 24, 2012 12:32 UTC (Wed) by cesarb (subscriber, #6266) [Link] (7 responses)

> what he said is that KVM needs (as in requires) hardware virtulaization extensions to work, while Xen does not

Yes, that is what I meant. I did not notice that "needs" could be read in an alternate way (as in "should have" or "is missing"); sorry for the confusion.

The problem is that not all systems are able to do hardware virtualization. You have older machines which are from before the virtualization extensions, newer machines where for some reason the virtualization extensions are disabled by the BIOS, and Intel processors where the same model might or might not have virtualization extensions (unless you know to look at http://ark.intel.com/ before buying).

An Introduction to Full Virtualization With Xen (Linux.com)

Posted Oct 24, 2012 14:38 UTC (Wed) by raven667 (subscriber, #5198) [Link] (4 responses)

At least from a server environment I don't think it's a good idea to run machines which are too old to have working VT instructions. It's cheaper for power/cooling to buy new, faster, more efficient machines and consolidate workloads than to keep running a larger number of slower, older, power-hungry hosts.

An Introduction to Full Virtualization With Xen (Linux.com)

Posted Oct 24, 2012 15:08 UTC (Wed) by gnb (subscriber, #5132) [Link] (3 responses)

Agreed for servers people sometimes need to run a VM on their desktop/laptop and there is surprisingly recent and otherwise OK hardware that lacks VT-x.

http://ark.intel.com/products/69669/Intel-Pentium-Process...

is a plausible laptop/low-end desktop CPU, 64-bit, came out this year, no VT-x.

An Introduction to Full Virtualization With Xen (Linux.com)

Posted Oct 24, 2012 16:38 UTC (Wed) by drag (guest, #31333) [Link]

Friends don't let friends buy Intel if they care about virtualization on low-medium range machines.

Intel intentionally disables features to create market segmentation. AMD does not do this and as such AMD is a superior processor for Linux desktop users that don't want to spend lots of money.

The idea of having the possibility of using Xen-style paravirtualized systems is lovely, but in practice it leaves a lot to be desired.

Two of the biggest reasons for using virtualization are to deal with legacy software that requires a specific configuration and being able to run Windows systems on Linux. Both of those things don't exist for Xen without VT hardware support.

AND if you can take advantage of using Xen PV without changing kernels or anything like that then you will almost always get better performance if you use something like LXC.

An Introduction to Full Virtualization With Xen (Linux.com)

Posted Oct 26, 2012 21:47 UTC (Fri) by jond (subscriber, #37669) [Link] (1 responses)

A gaggle of J. Random Developers with laptops that lack VT is not what is sustaining Xen. It must be something else.

An Introduction to Full Virtualization With Xen (Linux.com)

Posted Oct 26, 2012 22:16 UTC (Fri) by dlang (guest, #313) [Link]

I believe it's primarily Enterprise Inertia

at one time Xen was "the way to do virtualization" on Linux, and enterprises that setup their networks at that time aren't willing to change.

Actually, I strongly suspect that most of those organizations are still running the OS versions that they installed on the systems, but because of the 'installed base', having Xen updates in new versions is 'important', even for those companies that aren't running the new versions (after all, they may want to, and it shows that they made the right decisions way back when)

An Introduction to Full Virtualization With Xen (Linux.com)

Posted Oct 26, 2012 21:46 UTC (Fri) by jond (subscriber, #37669) [Link] (1 responses)

I find it hard to believe that any of those reasons you cite are genuine reasons that people would run production stuff on Xen. I'm sure such reasons exist, I just don't know them.

An Introduction to Full Virtualization With Xen (Linux.com)

Posted Oct 27, 2012 1:25 UTC (Sat) by ixs (subscriber, #47170) [Link]

Xen offers considerably lower latencies for both disk IO as well as network IO.
Even with PV drivers kvm is not even playing in the same league whenever we test this.