Virtual Machines and CentOS 5.0 [Linux]

Kristian Eklund kristian at
Tue Jun 24 05:58:39 CDT 2008

Hi Scott,

> What performance experiences have list members had with running virtual 
> machines (VMWare Server, VMWare ESX, etc) versus running applications 
> directly in the host OS?

I have purchased one of those servers myself with one quad-core CPU and 
8 GB RAM. We use it for lab-purposes in my small security consulting 
firm, and we have had no issues running several virtual machines at the 
same time. We have not tried to max it out yet.

> What are your experience differences between Xen and VMWare?

I use CentOS 5.1 and VMware Server.

The difference between VMware and Xen is the type of virtualization.

With Linux-guests you can para-virtualize them with Xen and achieve 
better performance (maybe 5-10% performance-loss) and with VMware you 
use full virtualization. Full virtualization could lead up to 20% loss, 
but I have never noticed anything like that. The CPUs today are so fast 
that even a 10% loss won't be noticable.

AFAIK Windows cannot be para-virtualized (yet) on Xen.

> What is the real difference between the free VMWAre Server and VMware 
> VSX[i]?   How about performance differences between these two products?

The difference between VMware ESX and VMware Server is that VMware 
Server has to be hosted on an operating system of your choice (Linux is 
perferred!). ESX is a commercial Linux-distribution from VMware that 
installs a very customized RHEL-distribution running on kernel 2.4.

Security differs since VMware has thought of this and tighten down their 
distribution very hard. They provide a software firewall and a 
possibility to only connect to the ESX via a seperate NIC. (I have done 
this with CentOS as well, if you want to know how I did it, contact me 
directly). If you feel confortable with CentOS you can shut down 
services you don't use and uninstall them.

The performance difference is maybe a few percent. I haven't tested 
this, but from what I've read elsewhere it's somewhere around that figure.

ESX also has functions to limit CPU-resources to virtual machines (i.e. 
Use max 1000 MHz). With VMware Server you can only limit the RAM.

> I have users who have been known to bring a physical machine or cluster to 
> its knees due to intensive CPU/RAM processing - not bad code, just the 
> product of their code.   How will a VM world possible help/hurt their 
> efforts?   I don't believe they will be making any specific CPU 
> instruction calls, for what that's worth.

If they need these amount of CPU-resources (and it's not a bug in their 
code) then I would not virtualize their machines.. If it's just a bug in 
their code ESX would be preferred to limit CPU-resources.

Kristian Eklund, Secure Networks (Swedish)
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