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RE: [Xen-users] Memory detection on a Supermicro P8SCi 4GB server

  • To: "Thomas Goirand (GPLHost)" <nospam.thomas@xxxxxxxxxx>, xen-users@xxxxxxxxxxxxxxxxxxx
  • From: "Petersson, Mats" <Mats.Petersson@xxxxxxx>
  • Date: Thu, 20 Apr 2006 12:34:51 +0200
  • Delivery-date: Thu, 20 Apr 2006 03:40:57 -0700
  • List-id: Xen user discussion <xen-users.lists.xensource.com>
  • Thread-index: AcZkYJZ+c4BgQpgySqS2BOlgi5bclAABNbTQ
  • Thread-topic: [Xen-users] Memory detection on a Supermicro P8SCi 4GB server

> -----Original Message-----
> From: xen-users-bounces@xxxxxxxxxxxxxxxxxxx 
> [mailto:xen-users-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of 
> Thomas Goirand (GPLHost)
> Sent: 19 April 2006 05:26
> To: xen-users@xxxxxxxxxxxxxxxxxxx
> Subject: [Xen-users] Memory detection on a Supermicro P8SCi 4GB server
> Hello,
> I have a server in production using a Supermicro P8SCi board. 
> It has 4GB in, and the BIOS shows 4GB. Xen 2.0.7 sees only 
> 3GB, and I have 1GB that is lost. Has any of you had this 
> problem, and is there a way to resolve it? Does Xen 3.0.2 
> resolves that issue?
>     Thomas Goirand, GPLHost LLC Manager

That's because Xen 2.0 doesn't support PAE - Page Address Extension. 4GB
of RAM doesn't actually fit in 4GB of address space, since the PCI
devices will need some address space too - and for those to work well
with 32-bit OS's, they need to be below 4GB. So what you get is that you
have some 3GB or so of RAM below 4GB, then PCI devices, and if your
chipset and OS supports this, you get whatever RAM is covered by the PCI
devices "shuffled" up above 4GB. The end result is that you need an OS
(and Hypervisor) that understands more than 4GB of memory space. This
means either PAE or 64-bit OS/Hypervisor. Note also that you need to
have same bitness for the hypervisor and the OS that runs on top of it,
so you'd need to upgrade the OS as well as the hypervisor to PAE if you
want 4GB of RAM to be visible. 

Xen 3.0 supports both PAE and 64-bit (to be very exact, 64-bit paging is
just another extension on top of PAE, so it's using the same mechanism
to address it's memory)


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