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I have a Linux rhel xen host which will be running many 32 bit guests. (The guests have to be 32bit). The server has 48gb of ram, and currently it runs a xen kernel but this only sees 16gb of ram on the dom0.

Is there a way to avoid this problem ? Each guest only needs 256-512mb

Not hopeful, but any suggestions would be great.

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Why do they have to be 32 bit :/ ? –  Lucas Kauffman Sep 20 '11 at 15:15
    
Is the rhel host 32bit too? –  becomingwisest Sep 20 '11 at 16:33
    
they have to be 32 bit guests because they are virtual staging servers of real 32 bit machines, and we need to keep them as close to the real deal as possible. –  Sirex Sep 21 '11 at 6:54
    
and currently yes, the host is 32 bit too. –  Sirex Sep 21 '11 at 7:05
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2 Answers

up vote 3 down vote accepted

The trick here is that the bitness of the Xen hypervisor and the bitness of the dom0 kernel and other guests can be different.

You can run a 64bit hypervisor and have a 32bit dom0 and guests.

Also of note, the dom0 doesn't manage the memory, but instead Xen the hypervisor does.

In your dom0 you will only see the memory that is allocated to dom0 by Xen. Xen the hypervisor (as long as it is 64bit (or even 32bit pae should work)) can handle the larger amount of memory. top will show the memory of dom0, which is what you checked, but xm top (or xentop) will show the amount of memory that Xen knows about. Check xm top (xentop) and see if Xen knows about all of the memory. If not, you should be able to install the 64bit version of the xen hypervisor (not that this is different than the dom0 kernel, which can still be 32bit - it is actually recommended that it is 32bit for performance reasons - see http://www.slideshare.net/xen_com_mgr/6-stefano-spvhvm

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ill look into this. 64 just on the host may be an option. I wasn't sure you could run 32bit domU's under a 64bit dom0. Do those 32bit guests need to be pae kernels (if addressing 256mb of the hosts 48gb each) ? –  Sirex Sep 21 '11 at 7:00
    
xentop shows 16gb of ram on the dom0 currently, on a 32bit xen kernel. Are you saying you can run a 64bit xen hypervisor on top of a 32bit kernel ?? –  Sirex Sep 21 '11 at 7:02
    
a colleague got this working. apparently we had to add "nolapic" to the kernel options, else it wouldn't boot, but now its all looking good. thanks ! –  Sirex Sep 21 '11 at 13:21
    
ah. downside is nolapic makes the 8cores disappear and presents just 1 to the o/s. bummer lol. –  Sirex Sep 21 '11 at 14:21
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According to http://www.redhat.com/rhel/compare/

[3] The "SMP" kernel supports a maximum of 16GB of main memory. Systems with more than 16GB of main memory use the "Hugemem" kernel. In certain workload scenarios it may be advantageous to use the "Hugemem" kernel on systems with more than 12GB of main memory.

[4] The x86 "Hugemem" kernel is not provided in Red Hat Enterprise Linux 5 or 6.

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