[Fedora-xen] Puzzled by Xen

[Peter Arremann]

On Monday 03 April 2006 05:59, Benjy Grogan wrote:
> Hello:
>
> I've read that Xen will allow servers that typically operate at 15% to
> 25% CPU capacity to operate at up to 80% with virtualization.  Does
> that mean that CPU usage will reach 80% instead of a lowly 15% to 25%?
>  Does CPU capacity equate CPU usage?  Instead of putting more work on
> different host OSes within Xen why not put all of that work in one
> single OS to achieve that 80% CPU usage?
>
> I've been reading up on Xen, and still this basic question puzzles me.
>  Is there something that I'm not getting about virtualization?
>
> Benji
>
> PS: To me CPU usage is when you look at the system monitor and see the
> load.

The purpose of Xen is to utilitze your servers better. If you had 4 identical 
servers that all ran at 10% cpu utilization, then if you put them together 
into one box using xen, you'll have theoretically 40% usage of 1 system and 3 
to do other things with. Xen of course has some overhead, so you will 
probably see 45% total usage or such. 

The reason why you want to do that is obvious - less servers needed to get the 
same work done. Power, air conditioning, space and so on all add to the cost 
of purchasing a server. 

If you can add all these services onto one server, you're better off doing so 
most of the time. But very often you will find, that it is not possible. What 
if your servers are part of different NIS domains? One OS image can only be 
part of 1 NIS domain at the time... What if each server is running a 
different instance of a program on the same port? There are lots of reasons. 
Most can be worked around, but over time the number and difficulty needed for 
those work arounds will add up and you will find it much easier to go with 
Xen.

Management is another reason to go with xen. You can very easily migrate OS 
images around as needed in xen. So if your workload changes, you can move 
some work to another server easily without having to do major re 
configurations in your OS. 

Security is also a good point. If you're running an app like apache that is 
fairly common and well tested, you're usually OK. Now add in some other app 
that is listening on the network but isn't hit directly by your customer 
(thing DB or custom apps). If you have only 1 instance of your OS running, 
both apps are on the customer facing network. If you run xen, you can play 
tricks and have only the apache portion exposed, while the communications 
with the other app is done over a private network that the users don't have 
direct access too.

Compatibility is yet another reason. You can run any xen based domU on any 
dom0 - at least in theory... Right now you'll run into issues with xen2 and 
xen3 version and stuff - but some day you'll be able to run Solaris x86 and 
Netbsd ontop of you fedora dom0.

In the end, Xen doesn't do anything you couldn't in a perfect open source 
world do in a single OS instance. It just makes things a lot easier and keeps 
you from doing a ton of extra work to get things running.

Peter.