[CentOS] new memory not getting regonized

Tue Aug 30 13:30:46 UTC 2011
Dennis Jacobfeuerborn <dennisml at conversis.de>

On 08/27/2011 09:12 PM, sylvan.dcunha at gmail.com wrote:
> Dear Dennis,
>
> Thanks a lot for the wise reply.. really did boost my knowledge..
> honestly was unware of the fact that dom0 is just like another VM ...
> Anyway I had never restricted dom0 mem and since my 4 vms were working fine
> with no issues
> i never bothered much.

Yes, this is different from KVM where the VMs really are just normal 
processes on the host system and the host system itself isn't a VM.

On a Xen system if you look at /etc/grub.conf you'll notice that it looks 
slightly different than on a non-virtualized system. Specifically you'll 
find the following line:
    kernel /xen.gz-2.6.18-164.el5

That's the actual hypervisor and really the host system and once started it 
will basically start dom0 and give it special privileges. So Dom0 and the 
DomU's all run on top of the actual hypervisor.

> It was only after I added more 32 gb to existing 32 gb i did realise the
> above issue..

Apparently dom0 has a 32G limit but that shouldn't be an issue unless you 
actually really require more than 32G specifically for dom0 and not the VMs.

> anyway I will try to restrict my dom0 to 1 GB ... and check it out.

Remember that the problems with the dynamic memory management are most 
likely fixed nowadays so the limitation is not strictly necessary. But then 
1G will probably be more than enough for dom0 so it doesn't really hurt either.

> but just still a litle confused why xm top & xm info shows 65gb
> and top , free and cat /proc/meminfo shows 32 gb

The xm tools show you the actual physical memory in the system while the 
/proc/meminfo shows you only the memory visible to dom0 which as I 
mentioned above is apparently limited to 32G.

Regards,
   Dennis