[CentOS] Centos 5.2 and Xen

Ruslan Sivak russ at vshift.com
Mon Jun 16 01:59:54 UTC 2008

Luke S Crawford wrote:
> 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.
If it's PAE, then I'm a bit confused, as they advertise it as "*Native 
64-bit hypervisor:* Scalability and support for enterprise applications"
>> 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
> windows.  
Well I have up to 4GB of run windows and I can have the other 4GB for 
dom0, so if I can get OpenVZ or linux vserver running on there, I can 
use that to run my linux VM's. 

Doesn't openVZ require a different kernel?  That would replace the Xen 
kernel wouldn't it?  Or is there a way to custom compile Xen+OpenVZ kernel?

I'm not too familiar with linux vserver, but my guess is you can't run 
it in Dom0 either...


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