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 don’t
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 shouldn’t have as many potential security
flaws, so shouldn’t 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 ‘t do this because it isn’t supported and
is rather old, but when I started setting up, CentOS didn’t include the drivers
I needed, and I didn’t need any newer features that weren’t already available.
Since my dom0 and VMs aren’t exposed to the outside world, I didn’t 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 don’t 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 don’t KNOW that KVM’s virtual machines
can’t run without a user logged in, but I don’t know how to make it do it, and
I don ‘t 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 I’m 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 I’m running Windows).
Dustin
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