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I have a server with 48GB or RAM on which I have installed RHEL5 with the xen kernel. I think maybe this is normal behavior, but the kernel only seems to be able to see 32GB. I looked around online, and I found very little. I think a significant data point is that xm info reports total_memory as 49142 (MB, I assume), which I suppose means that I can utilize the 48GB for my VMs?

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If xm info reports all of the RAM then you should be able to assign it out to your domU's without a problem. I typically set the dom0_mem setting on the grub.conf boot line to limit the RAM the dom0 can utilize and save the rest for the domU's. I couple this with setting dom0-min-mem to 0 in xend-config.sxp to keep it from ballooning larger.

I'm not aware if there is a 32GB limit to the dom0, but I've not had a system with that much RAM to be able to test it either. Xen requires a PAE-based kernel build so it should be able to address the RAM.

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Thanks, Jeremy. I often forget that the dom0 is just another domain, although it has some extra privileges. I'll feel a little better if I could find a reference to a 32GB limit on memory in Xen domains, but that does seem to be the case. –  Chad Huneycutt Jul 27 '09 at 18:57
    
Correct... The Xen hypervisor is actually loaded by GRUB and then it starts dom0 as the privileged domain. The domU is really no different from the dom0 except it's unprivileged. I use the dom0 only for a management backplane so usually limit it to 1 vCPU and 256M RAM. –  Jeremy Bouse Jul 27 '09 at 20:16
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