[CentOS] Centos 5.2 and Xen

Luke S Crawford lsc at prgmr.com
Sun Jun 15 22:45:48 UTC 2008

Ruslan Sivak <russ at vshift.com> writes:
> > running vmware under a xenU guest wouldn't lift any ram limit
> > imposed by the xen kernel or dom0.


> The 4GB limit is artificial, and only applies to the vm's started
> using their closed source XenSource.  The host OS is most likely
> CentOS 5, and sees the whole 8GB (although it's not x64, so I'm
> guessing they use PAE or something.)

It is PAE.

> I only need 8GB of ram support, and no other features that are offered
> in XenStandard, so it seems kind of a waste to pay $1k per server for
> that. If another virtualization technology was installed on that OS,
> you can get the use of the other 4GB, and if not, I can always run my
> apps on Dom0, although I'd prefer to not install too much stuff on
> Dom0.

First,  The Dom0 OS runs as a guest of the Xen hypervisor-  it is just
a guest that happens to have access to the PCI bus as well.  The Xen
hypervisor still controls what ram and CPU all domains including the Dom0,
 can see;  if the xen kernel is limiting you to 4G ram total, that 
limit will apply in the Dom0 as well.

Also, you are not going to be able to run a virtualization technology that
uses the hardware virtualization support from within a Xen guest, even
if that Xen guest happens to be the Dom0.   The Xen hypervisor
controls access to those instructions.  

You can run virtualization technologies that don't require HVM-   OpenVZ and
linux vserver will both work fine.  Heck, you can do that within an 
unprivileged Xen DomU, but that won't help you if you want to run

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