Xen users - Oct 2010 - Trying to get a good grasp on what to expect from Xen

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.

In summary, I''d like to deploy a Virtual Server so that I may run guest
Linux machines for mostly web hosting, file servers, databases (maybe SMTP
at a later time).  I will be buying new hardware to build my own rack-mount
unit so I am not constrained by processor, memory, etc as those have not
been acquired yet.  However, for the record, I''m looking perhaps to go
with
the Intel Quad Core Xeon with at least 8 GB of RAM to start.

My problem at this point is this:  There are some fundamental questions I
have about Open Source Xen Hypervisor that I have not been able to find
definitive answers answers to and I hope someone out there may provide some
insight:

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 ...

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?

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.

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

4.  Are there substantial performance differences with Paravirtualization vs
HVM?  Which is best in the long run for performance, stability and
portability?

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.

6.  Is it possible or advisable to run the domain0 OS from a RAID 5 storage
device?  What about guest OSes?

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?


This is my first virtual solution deployment and so I''m still testing
the
waters here.  I hope some of the answers can help me decide on a solution
and approach that I can build and grow with.

Regards,
Manny


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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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On Wed, Nov 10, 2010 at 2:57 PM, Craig Miskell <craig.miskell@opus.co.nz>
wrote:
>> 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.
note that RAID5 is rarely "the best performance possible".  for that,
check RAID10 or at least RAID1 (or RAID0, if you don''t care about data
loss)

-- 
Javier

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