[CentOS-virt] Xen/VMWare Server comparison and "best Xen practices"?

Nicolas Sahlqvist nicco77 at gmail.com
Mon Oct 15 12:26:28 UTC 2007

On 10/15/07, Kai Schaetzl <maillists at conactive.com> wrote:
> I'm wondering what others who have already used the Xen kernel for DomUs
> and the (free) VMWare Server can say about the comparison in actual
> day-to-day operation. I've only started playing with Xen on CentOS 5 and
> I've been running VMWare Server only on Win2k3 servers so far, so I'm
> missing direct comparibility. I find that VMWare is highly reliable and
> flexible, but needs a good portion of RAM and CPU for working nicely. Xen
> seems to be less reliable, but more responsive on low ressources. It's
> also more flexible when you want to move it around as it is all stored in
> a single file and the config file takes only a few options.
> Xen is supposed to give "almost the same performance" as if not
> virtualized. I get confusing figures from the Virtual Machine Manager.
> On my test machine the Dom0 takes about 9% of the CPU with one VM when
> both are mostly idle. Top says that 3% of that is taken by the X-Server,
> most of the rest is shared by two python processes (which belong to Xen I
> suppose) and xenstored. (The test machine is on a single Athlon 2500+ or
> around that, not sure about the exact speed.) But regularly one of the
> python processes grabs another 10-15%, so the whole CPU utilization goes
> up to around 20%. Although the single VM is idle at that time and shows
> 0.0x%. So, even when idle the whole CPU utilization zigzags between 10 and
> 20%.
> I found that when I close the VM console that drops to 2-3%, so that
> python process is obviously related to the VM console. The interesting
> thing is that when I then reopen the console from the VM manager it keeps
> going at about 3% and doesn't go up to the earlier 9%. But it still
> zigzags between 3 and 11% then. All the figures have been taken from the
> VM manager. The %us count in top seems to stay at 3% all the time.
> With less reliable I refer to a filesystem problem that seems to occur
> sometimes after rebooting/shutdown. Sometimes after rebooting there either
> is no filesystem file anymore (which is not healable, of course) or the
> kernel panics with a filesystem problem on first boot or reboot, but may
> boot just fine after a second or third try. The problem that the
> filesystem file is just gone seems to happen only when I reboot/shutdown
> from within the console with the shutdown/reboot command. For instance it
> can happen if I let it do the first reboot after I installed the OS on the
> VM. When I "xm shutdown" or CRTL+ALT+DEL or hit the shutdown button on the
> Virtual Machine Manager it does not *seem* to happen (maybe I just didn't
> shutdown often enough to see it happen there as well.) I've already lost
> several testing VMs because of this.
> I wonder if this problem might happen because I use the option of not
> allocating all space in the filesystem file right-away. I also wonder if
> performance might be better if it wouldn't need to grow.
> So, are there recommended best practices for Xen VMs like "always use
> partitions", "always allocated whole space at once", "never shutdown from
> the console window", "never use virtual machine manager" or some such?
> Kai

Hi Kai,

I have read that Xen is supposed to only take a few % without
virtualization support in the CPU, in my experience this is not
entirely true. I have experienced good CPU performance with degraded
I/O performance on Xen compared to the physical environment although
sufficient hardware resources were provided. You can read more about
other tests done in this regard, URL:


I've shutdown Xen 2 and 3 from within the console (shutdown -h now),
xm shutdown ... or xm destroy ... and I never got a crashed file
system, all environments on LVM partitions on Intel and AMD hardware.
I don't use the I use the option of not allocating all space in the
file system file right-away so it could be this or the image file, or
why not the combination? ;) Perhaps you can try out some combination
and let us know how this turns out..


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