Re: [Xen-users] New user question...

[Daniel Hulme <dh286@xxxxxxxxx>]

> I am new to virtualization and even newer to Linux.
Welcome to the crowd, in both cases. Lots of new users are put off Linux
because they have strange hardware that the drivers don't like and they
feel too overwhelmed to do the actions needed (sometimes simple,
sometimes harder) to make it work. I wish you luck in your explorations.

> The FAQ and documentation for Xen seem to presume a certain level of 
> prior low-level Linux and/or Xen knowledge.  (None of which I have...)
Well, everyone tries their best to make it as easy to read as possible,
but no easier. Xen is targetted at people who are comfortable
recompiling kernels and setting up new hard disk partitions, and the
documentation reflects this. With the increasing availability of Xen
packages in distributions[1] this is starting to change, and we will
have to keep editing the docs to reflect this, but at the moment you
will find it quite hard to set up without prior knowledge of what goes
on inside your Linux system.

> Can someone help answer up a few questions for me?
I can try.

> -- In the VWware world, each domain has the full illusion a system to 
> it's  self.  I get the idea that this is not the case in Xen, but how 
> can this be while still enforcing the integrity of each domain?
You are correct. Each domain in Xen realises that it doesn't own the
computer, but it can't access the whole computer, just as a user process
in a modern operating system is only allowed to write to its own memory
and not that of other processes. Xen does clever things with page tables
to ensure that each domain can only see its own memory: this provides
the speed benefit that only page table updates need to be verified by
the hypervisor (unlike in fully virtualized systems where each memory
access must be checked), but allows the same degree of protection.

> -- In the VWware world, I/O (Disk, Network, HID, VGA, etc..) is
> handled  by "virtual hardware devices" that can be assigned to each
> domain at  will, and are in turn somehow securely mapped and shared to
> their real  world counterparts.  Is this the case in Xen?
It is partially the case. Disk and network are virtualized by two-part
drivers. The 'frontend' driver sits in the user domain and pretends to
be a normal driver, but instead of talking to hardware, it talks to the
'backend' driver, which sits in domain 0 or a special 'driver domain'
and relays instructions to the hardware. We hope to add high-performance
paravirtualization of graphics hardware in the future, but it is hard.

> -- In the VWware ESX server product, identical memory pages among 
> similar domains can be transparently consolidated to free up more 
> physical RAM for the domains to use.  Is this also the case in Xen?
I'm not sure, but AFAIK it is not. Xen is designed with hosting
companies in mind. They want to charge users for the amount of memory
(etc.) they use, and sharing the memory like that would make that much
harder. It would also cause problems when domains wish to make changes
to transparently shared pages if there is not enough free memory to copy
them apart.

I hope I have been some help.
Daniel

1] http://wiki.xensource.com/xenwiki/DistributionSupport

-- 
Stop the infinite loop, I want to get off!     http://surreal.istic.org/
Paraphernalia/Never hides your broken bones,/ And I don't know why you'd
want to try:/ It's plain to see you're on your own.        -- Paul Simon
  The documentation that can be written is not the true documentation.

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