[lug] virtualization first boot question

karl horlen horlenkarl at yahoo.com
Sat Sep 18 14:21:37 MDT 2010

> libvirt will abstract a lot of the underlaying system
> commands.  If you
> want to know more, then you will want to look at the man
> pages for
> tunctl, brctl, qemu, ifconfig, iptables, and the xm
> command.  I got
> tired of running qemu manually so I wrote a python script
> that manages
> VMs for me.

got it

> Both dom0 (the host OS) and the VM will have to run Xen
> aware kernels if
> you want maximum performance.  If the guest OS does
> not run a Xen
> kernel, then Xen will default back to fully emulated HVM
> mode which is
> slow (10-15% overhead).

got it

> Paravirtualization is a technique where the host OS and the
> guest OS
> will cooporate with each other to perform functions that
> are difficult
> to emulate on the x86 CPU.  When you run a VM in PV
> mode, the VM is much
> smarter and knows that it is a VM.  The guest OS has
> to be ported to the
> hypervisor in order for PV to work.

that helps a lot.  in PV you are not just cramming a guest OS into an emulation sandbox that looks like a real system to the guest OS.  the guest OS has to be changed itself to work presumably more efficiently within is VM.

> You have them straight.  Xen isn't the only VM
> technology that supports
> Paravirtualization.  VMWare, Virtualbox, and KVM also
> can do it.  If you
> go into the kernel source and do make menuconfig you will
> see an option
> called Paravirt Ops.  If you click that on, you can
> tell the kernel to
> load support for a bunch of different hypervisors.

cool.  by sharing that you helped me see at a lower level how it relates to the the kernel.

> That's also correct.  Xen and KVM have different
> things that have to be
> in the kernel.  Xen changes the kernel fundamentally
> at it's lowest
> levels (which is why it's not in the official Linux
> kernel).  KVM is 3
> .ko files that you load with modprobe.

more understanding at lower levels and why i imagine people might prefer kvm over xen.

> I prefer KVM over Xen.  Xen changes the kernel
> fundamentally to deal
> with its guests, and from what I gather, most of the kernel
> developers
> despise it.  KVM allows a user space program to use
> the processor VM
> extensions.  This means that the emulator has to be
> ported to use KVM. 
> After that, the kernel runs the VM as if it was just
> another process on
> the system.

even better :)

> Virtualbox is hosted and runs on top of a booted OS. 

good to know.

> It needs the host
> to provide video, keyboard, and mouse input.  For
> virtualizing servers,
> Virtualbox is not what you want.  

got it.

thanks a lot for taking the time. i appreciate it.


More information about the LUG mailing list