Re: [Xen-users] Trying to get a good grasp on what to expect from Xen

[Craig Miskell <craig.miskell@xxxxxxxxxx>]

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Hi,
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Anyway, to answer your questions:
Manny T wrote:
> I've been doing researching for about two weeks now for a server
> virtualization solution for my small business and am at a point where
> there is some key information I need to move forward.
> 
> My understanding is that Xen Hyervisor is a "bare metal" solution that
> will offer optimal performance, in most cases, over other solutions that
> run inside a host OS -- if so ...
Yes, it is a bare metal solution, otherwise known as a "type-1" hypervisor.

> 1.  With Hyervisor, can I choose between Paravirtualization or Hardware
> Virtual Machine (HVM) or will I be forced down a particular path because
> of the nature of Hypervisor?
With Xen, running on processors with the Virtualisation extensions, you can do
either para-virtualization or full virtualisation, at your discretion.

> 2.  I've read that Paravirtualization usually requires some
> modifications to the guest OS kernal.  With the latest version of Xen
> and Linux available (Suse 11.2 or 11.3) is this still the case?  I would
> rather not have to modify the kernal or anything else in the hopes of
> maintaining portability should I have to move a guest OS at a later time.
Yes, this is still the case.  Para-virtualization is by definition where the
guest is aware of the fact that it is virtualized and behaves differently so as
to work better (more efficiently).  "Modifications" simply means a version of
the kernel compiled with different options, and most distributions include such
a variant with a slightly different name.  In the SuSE case, I believe its just
a package named like kernel-xen-<version>-<arch>.rpm (as opposed to just
kernel-<version>-<arch>.rpm).  But I don't use SuSE, so can't be sure.

When installing your paravirtualized guest, you choose/install the
paravirtualized kernel instead of the normal kernel and your job is done.

> 3.  I plan to purchase a processor that supports Hardware
> Virtualization, will this allow we to use HVM?
Yes

> 4.  Are there substantial performance differences with
> Paravirtualization vs HVM?  Which is best in the long run for
> performance, stability and portability?
Paravirtualization is usually (yes, a weasel word :-))faster than HVM, because
the guest kernel doesn't do things that cause big expensive traps/exceptions to
occur that need to be emulated by the host, and can handle I/O through specific
drivers that know they're virtual, rather than having the host emulate standard
physical devices.

However, these days there are things called paravirtualized drivers for HVM
guests.  The guest is truly HVM, but has a fake PCI device presented by the
host, with a driver that knows it's virtualized and uses said PCI device very
efficiently; this helps out the performance of I/O, narrowing the gap, without
having to modify the guest kernel.

> 5.  In the interest of getting the best performance possible, is it
> possible or advisable to install and run Hypervisor from a local RAID 5
> storage device?  This RAID 5 disk would be where I also plan to store my
> guest OS files.
Possible: yes.  Advisable: depends on size and number of guests, I/O load from
said guests, the number of disks in the RAID, specs of said disks and the I/O
backplane etc.

> 6.  Is it possible or advisable to run the domain0 OS from a RAID 5
> storage device?  What about guest OSes?
There's no technical restriction from the perspective of Xen, as long as there's
"some disk space" available.

> 7.  I also see that with Suse 11.x, there is an option in YAST to
> install Xen.  Will this install Xen Hypervisor with the current Linux OS
> as domain0?  What exactly is going on here?
No idea :)

- --
Craig Miskell
Senior Systems Administrator
Opus International Consultants
Phone: +64 4 471 7209
> As I clarified for a co-worker just a few days ago - "I didn't say it
> was your fault.  I said I was going to _blame_ you."
Randy the Random on a.s.r.
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