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RE: [Xen-users] RHEL xen vs kvm



 

 

From: xen-users-bounces@xxxxxxxxxxxxxxxxxxx [mailto:xen-users-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of Grant McWilliams
Sent: Thursday, April 22, 2010 02:43
To: Jeff Sturm
Cc: Arpan Jindal; Xen List
Subject: Re: [Xen-users] RHEL xen vs kvm

 

At some point (and we're fast getting there) we'll be able to apply Linus' quote about the kernel not mattering to hypervisors. About now the hypervisor is starting to not matter and how you manage your VMs is the real reason to choose which system you use.

Currently I only use Paravirtualization because it's about as fast as bare metal (databases are the worst for virtualization and mysqlbench shows performance within 1% of bare metal) and if you set them up with their own kernel inside the VM disk it looks and acts like a real Linux server. The other mode with Xen is HVM which is full virtualization and is necessary to virtualize Windows. KVM does a better job of this then Xen and is faster for full virt. However KVM isn't as fast as Xen PV even with KVM PV drivers. It all depends on what your needs are. If we go by the Xen summit slides the future of Xen is in hybrid virtualization which uses hardware virtualization for everything the hardware supports and then uses paravirtualization for everything else. This will be the best of both worlds (HVM and PV). I don't see KVM moving away from what it's doing (using Qemu for a lot of stuff, Hardware VT and paravirtualized network and disk drivers). How much of a difference this will make I'm not sure.

Here's my thoughts.

If I were primarily virtualizing Windows I'd use KVM.
If I were primarily virtualizing Linux I'd use Xen.
If I was using a bunch of old 3.4 Ghz Dual Core Xeons (I am) I'd use Xen.
If I was wanting to nest VMs I'd use AMD CPUs and KVM (for now).
If I wanted the most pain free path to keeping my hypervisor updated I'd use KVM.
If I was doing desktop virtualization (local login, not network logins) I'd use KVM or VirtualBox
If I wanted the most tried and true enterprise hypervisor out there and didn't want to use VMWARE then I'd use Xen. Citrix Xenserver, VirtualIron, Sun SVM (one flavor), Oracle Virtual Machine and Amazon EC2 are all based on Xen.


It might look like I lean toward KVM from this list but I still prefer Xen in most cases because of category 2.

Grant McWilliams

 

I donât spend the majority of my time messing with this, but I do follow the list, and I can share my thoughts and experience. My thoughts are that yes, a hypervisor should be better, it is compact and shouldnât have as many potential security flaws, so shouldnât need upgraded as often. Simple or not, each upgrade brings potential new problems, so stick with a stable hypervisor unless you need to fix a security issue, are seeking new features, or are squashing a bug. My experience is I am running 4 Windows VMs on the version of Xen included with Fedora 8. Obviously most people can ât do this because it isnât supported and is rather old, but when I started setting up, CentOS didnât include the drivers I needed, and I didnât need any newer features that werenât already available. Since my dom0 and VMs arenât exposed to the outside world, I didnât need to worry as much about the latest security updates. My reasons may be way out of date now, but at the time, I was able to get better performance in Windows out of Xen using the GPLPV drivers, and unless additional drivers have been developed for KVM in the past few years, I donât see how fully virtualized KVM machines could be faster than Xen HVM machines using PV drivers. In addition to being able to get better performance in Windows on Xen, I found it easier to automatically start everything in Xen (I donât KNOW that KVMâs virtual machines canât run without a user logged in, but I donât know how to make it do it, and I don ât know how to make a user automatically log in, nor do I think I should have to, nor do I want my VMs associated with a user), perhaps this is only because I had more experience with Xen and spent less time researching KVM than I could have (but then, why keep researching when the solution Iâm more familiar with performs better as well?). Anyway, that sums up my experience and the reasons for making the choices I did (F8 even though it was toward EOL and Xen even though Iâm running Windows).

Dustin

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