Zen first or CentOS first?
 

My understanding is Zen is a hypervisor running beneath all OS. However, why would this site says I can install Zen after CentOS is installed?

Also, according to the official documentation for Zen 3.0 here, I have either one of the following two options for install Zen. Well, in order to do this don't I have to have a running OS in order to execute these commands?

Install from Binary Tarball
# tar zxvf xen-3.0-install.tgz
# cd xen-3.0-install
# sh ./install.sh

Install from RPM
# rpm -iv rpmname

Is it simply the original installed OS will be decoupled from the hardware once Zen is installed and automatically become Domain 0?

BTW, guess this Red Hat Enterprise Virtualization for Servers is based on Zen?
https://www.redhat.com/apps/store/rhev/servers/

On RHEL (and hence CentOS), installing Xen involves installing a different version of the kernel - hence you would need to install the base OS first and thereafter do a kernel update (and update your grub).

Cheers,
Yas

Hi. Thought RHEL 5 and CentOS 5 uses Xen (RHEL 6 I've read is going to KVM). When I goto http://www.centos.org, I see the following statement "Virtualization provided by the Xen hypervisor with Virtual Machine Manager and libvirt." under the CentOS 5 Releases. Does that mean this particular version has been compiled to use with Xen, or Xen has been bundled with it already?

Hi

Up to RHEL 5.3 (and hence CentOS 5.3) - Xen is used.

From RHEL 5.4 onwards KVM is used.

Xen is not bundled with the base OS - you need to do base install; and thereafter install the Xen packages as well as the Xen kernel.

Thereafter a reboot into the Xen kernel and you're good to go. Do a bit of searching on the web and you'll find plenty of step by step guides for setting up Xen VM's on RHEL/CentOS.

libvirt refers to RHEL's GUI based virtualization manager - which can be used to setup Xen or KVM based VM's.

Hope this helps

Cheers,
Yas

No, it's based on KVM, Xen is out of the picture

alli_yas, thanks for the reply. Just bought a book on RHEL 5 today which shows pretty much all the steps on how to install the Xen, and how to use both Virtualization Manager and the command line xm to mange the VMs.

Just like what you said you'll have a base OS, then use yum to install both Xen and the modified kernel RHEL, and restart into the modified kernel RHEL, which will be the Dom0 OS I think.....

I also recall reading on Xen's site that the VMs will make calls to the Dom0 OS to access hardware. Would Dom0's OS then make calls to the original OS? What if this Dom0 OS fails?

Hi there

From my understanding of Xen:

OK I think you're confusing yourself here into thinking that Dom0 and the host OS (what I think you term the "original OS") are separate entities. When you install the Xen kernel; and boot RHEL/CentOS - this is actually Dom0.

From what I understand, the Xen versions of the kernel contain code that actually does the mappings between the Hypervisor and the hardware. So long story short; your original/host OS is Dom0 ;)

Well, as per the above explanation - if your host OS or domain OS crashes for some reason, well so will your VM's. If you think about it, the kernel facilitates the operation of Xen virtualization and thus if the kernel fails/panics; your VM's should crash as well.

Hope this helps

Cheers,
Yas

alli_yas, thanks for the additional information. So the Host OS is essentially Dom0. Now here are my questions and observations:
1) If the original OS (before Xen is installed) becomes Dom0, that means the very first OS I installed must be a modified Linux version that would work with the Xen software. I guess CentOS 5 and RHEL 5 are already ready and I don't have to download a special version?
2) For VM's ESXi, it's a true Type 1 baremetal hypervisor in that no VMs will depend on any OS. Technically if ESXi fails, then also all VMs will fail. However, given that ESXi's surface area is so small, I guess the chance is quite slim.
3) For Xen, it's also labelled as a Type 1 baremetal hypervisor. However, in this case Xen itself still needs a host OS (Dom0) in order communicate with the hardware. Is there any reason why it's not made to be like the ESXi methodology?
4) Xen's online documentation gives an diagram of how it sits beneath all the Doms (incl Dom0). I thought initially DomUs all communicate with hardware via Dom0, then the Xen hypervisor, before hitting the hardware layer. However, given "the Xen versions of the kernel contain code that actually does the mappings between the Hypervisor and the hardware", I guess in reality DomUs all communicate with hardware via Dom0 (since Xen relies on Dom0 to make the hardware access)? In this case, what dos Xen do actually?

I think where your getting confused is where Xen fits in.
The operating system comes in two flavors - One which supports virtualisation and one that doesn't. The standard OS (your original) does not support virtualisation. The Xen enabled one does.
Installing the kernel-xen package enables virtualization.
In this mode, the operating system shows as Dom0 in virtualisation manager.

True, but technically ESXi is an operating system.
You can get the same/similar effect on Linux by just doing the base install. Xenserver is an example of this approach.

I think your seeing Xen as a three tier model: OS - Xen (Dom0) - VM (DomU) i.e. separate from the operating system.
In reality its an integral component that enables the kernel to function as a hypervisor: Xen OS (Dom0) - VM (DomU).

In this regard, its the same as most hypervisors (such as ESXi or KVM).
All of these are essentially Dom0.

HTH

You mean if I already have a non virtualization supported OS of CentOS and RHEL, I have to install the kernel-xen, while I install Xen? This way the kernel-xen will become Dom0? In this case, what happens to the original non Xen kernel RHEL or CentOS? Can I remove them?

How do you mean? Sorry can you elaborate?

kernel-xen gets installed as part of the the Xen Virtualization package group, which also installs all the management tools.

Like any previous kernel, their files are still stored in /boot and as an option in the grub boot menu. If you don't need them they can be removed.

XenServer is an example of a pre-packaged Xen server solution.
Once booted it looks very similar to the ESXi startup screen.
It's available in open source form at http://www.xen.org/products/cloudxen.html, or as a commercial product at http://www.xenserver.com.

HTH

sp-support's posts above pretty much answer your questions in a similar fashion that I would.

To add on to the above, understand that the kernel is the core of the OS - and thus when you install the Xen kernel; you're installing a standard RHEL kernel that has been modified and coded to support the Xen packages/applications. However, just like any other kernel upgrade that you do, it doesn't mean that your old kernel is deleted/destroyed (unless you specifically do this) - you can still reboot the older (non-Xen) kernel; where you won't be able to start up your VM's.

Thanks
Yas