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Old 05-12-2012, 05:57 PM
Theo Band
 
Default XEN or KVM - performance/stability/security?

On 05/12/2012 12:46 AM, Gordon Messmer wrote:
> A late reply, but hopefully a useful set of feedback for the archives:
Well let me share my experience as well.
> On 04/20/2012 05:59 AM, Rafał Radecki wrote:
>> Key factors from my opint of view are:
>> - stability (which one runs more smoothly on CentOS?)
> I found that xenconsoled could frequently crash in Xen dom0, and that
> guests would be unable to reboot until it was fixed. I also found that
> paravirt CentOS domUs would not boot if they were updated before the
> dom0. In short, Xen paravirt was very fragile and troublesome. I never
> tested Xen with hardware virtualization.
>
> I have had no such problems with KVM. In my experience KVM is much more
> stable than Xen paravirtualization. Xen HVM probably would suffer at
> least some of the same problems.
I have some machine that were very unstable under load (max uptime some
weeks, then a crash). They were running CentOS5 with XEN kernel. First I
thought it was hardware related, but once a non-Xen kernel was loaded
and I migrated the VMs to KVM the machines are rock solid.
I must say I still have two machines running Xen and they have no
problem the last year. So it's probably also related to the specified
hardware configuration.

Theo

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Old 05-12-2012, 06:38 PM
Steve Thompson
 
Default XEN or KVM - performance/stability/security?

On Sun, 6 May 2012, Pasi Kärkkäinen wrote:


with "fork performance" I assume you're comparing Xen PV to KVM ?
Yes, PV has disadvantage (per design) for that workload, since the hypervisor
needs to check and verify each new process page table, and that has some performance hit.
For good "fork performance" you can use Xen HVM VMs, which will perform well for that workload,
and won't have the mentioned performance hit.


I used both PV and HVM VMs. I don't have the details to hand at the
moment, but KVM was superior to both. PV drivers where applicable. I have
been running KVM for about 15 months now, with 30 VM's on one host and 38
VM's on another. It has been solid; no problems, but unfortunately I had

problems with Xen.

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Old 05-12-2012, 06:42 PM
Dennis Jacobfeuerborn
 
Default XEN or KVM - performance/stability/security?

On 05/12/2012 12:46 AM, Gordon Messmer wrote:
> A late reply, but hopefully a useful set of feedback for the archives:
>
> On 04/20/2012 05:59 AM, Rafał Radecki wrote:
>> Key factors from my opint of view are:
>> - stability (which one runs more smoothly on CentOS?)
>
> I found that xenconsoled could frequently crash in Xen dom0, and that
> guests would be unable to reboot until it was fixed. I also found that
> paravirt CentOS domUs would not boot if they were updated before the
> dom0. In short, Xen paravirt was very fragile and troublesome. I never
> tested Xen with hardware virtualization.
>
> I have had no such problems with KVM. In my experience KVM is much more
> stable than Xen paravirtualization. Xen HVM probably would suffer at
> least some of the same problems.
>
>> - performance (XEN PV/HVM(with or without pv drivers) vs KVM HVM(with or
>> without pv drivers))
>
> PV drivers will make some difference, but the biggest performance
> difference you'll see is probably the difference between file-backed VMs
> and LVM-backed VMs. File-backed VMs are extremely slow. Whichever
> system you choose, use LVMs as the backing for your guests.
>
>> - security
>
> There have been bugs that allow guests to escalate privileges and access
> host resources, but they're relatively few. I don't think there's a
> significant difference between the two in this area.

sVirt mitigates this danger somewhat on the host side so even if you run
into such an issue it is very hard to utilize such an export.

Regards,
Dennis
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Old 05-16-2012, 09:47 PM
"Luke S. Crawford"
 
Default XEN or KVM - performance/stability/security?

On Fri, May 11, 2012 at 03:46:43PM -0700, Gordon Messmer wrote:
> A late reply, but hopefully a useful set of feedback for the archives:
>
> On 04/20/2012 05:59 AM, Rafał Radecki wrote:
> > Key factors from my opint of view are:
> > - stability (which one runs more smoothly on CentOS?)
>
> I found that xenconsoled could frequently crash in Xen dom0, and that
> guests would be unable to reboot until it was fixed. I also found that
> paravirt CentOS domUs would not boot if they were updated before the
> dom0. In short, Xen paravirt was very fragile and troublesome. I never
> tested Xen with hardware virtualization.

This particular problem was fixed some time ago, it hasn't happened
to my (many) dom0s in more than a year.

The RHEL5 Xen dom0 was garbage until 5.3 or so. To the point where I'd
compile my own and deal with the pain of using a non-rhel kernel with
a rehl userland.

Stability has improved vastly.

> > - performance (XEN PV/HVM(with or without pv drivers) vs KVM HVM(with or
> > without pv drivers))
>
> PV drivers will make some difference, but the biggest performance
> difference you'll see is probably the difference between file-backed VMs
> and LVM-backed VMs. File-backed VMs are extremely slow. Whichever
> system you choose, use LVMs as the backing for your guests.

My experience has been that using qemu for disk has something of a
multiplier effect; e.g. it makes slow spinning disk noticably
slower. The paravirtualized drivers help immensely in that regard.

(how are the paravirt drivers in KVM these days? I have a server
full of kvm guests running some ancient version of ubuntu I will be
moving to RHEL6 shortly.)
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Old 05-17-2012, 08:00 PM
Gordon Messmer
 
Default XEN or KVM - performance/stability/security?

On 05/16/2012 02:47 PM, Luke S. Crawford wrote:
> (how are the paravirt drivers in KVM these days? I have a server
> full of kvm guests running some ancient version of ubuntu I will be
> moving to RHEL6 shortly.)

Since RHEL guests have the virtio block drivers built-in, I never get
around to benchmarking them against non-virtio block devices.
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Old 07-08-2012, 03:47 PM
Pasi Kärkkäinen
 
Default XEN or KVM - performance/stability/security?

On Tue, Jun 05, 2012 at 01:50:30PM -0700, Gordon Messmer wrote:
> On 06/04/2012 11:36 PM, Pasi Kärkkäinen wrote:
> >
> > Xen PV has been rock solid for me
>
> Maybe, if we ignore the fact that you seem to be familiar with the
> problem of xenconsoled failing and preventing guests from booting.
>

The el5 xenconsoled bug:

1) Affected only 32bit (i686) hosts running el5 Xen
2) It was fixed years ago on upstream Xen
3) It is already fixed in rhel5/centos5
4) It didn't affect 64bit (x86_64) hosts running el5 Xen
5) It was easy to workaround by killing+restarting xenconsoled


> > Xen is supported by Red Hat support in RHEL5.
>
> Yes, and RHEL5 will be supported for several years. However, there does
> not appear to be a plan to support Xen in the future, after RHEL5
> expires. It would be irrational to invest time and money into training
> on Xen with no expectation that those skills will remain valuable in the
> future.
>

There are other Xen-based virtualization solutions out there aswell with full support.

-- Pasi

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Old 07-13-2012, 10:52 PM
Gordon Messmer
 
Default XEN or KVM - performance/stability/security?

On 07/08/2012 08:47 AM, Pasi Kärkkäinen wrote:
> There are other Xen-based virtualization solutions out there aswell with full support.

That's true, but I'm guessing that a lot of people on this list are here
specifically because they're not paying for support.

Whether that's true or not, if you're deploying virtualization *on
CentOS*, KVM is really the only rational choice. Xen hosting support is
no longer a part of new releases.


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