[CentOS] CentOS 5.4 x86_64 only detects 32GB RAM while Fedora x86_64 correctly lists 128GB

Pasi Kärkkäinen pasik at iki.fi
Tue Dec 8 13:41:36 UTC 2009

On Mon, Dec 07, 2009 at 01:05:04PM +0100, Diederick Stoffers wrote:
>    Hi,
>    We have a new 24-core Dell PowerEdge R905 server with 128GB's RAM. The 64
>    bits version of Fedora 12 lists the correct amount of 128GB, CentOS only
>    finds 32GB (and so does Scientific Linux). I would much prefer to use
>    CentOS (most of the software we use is specifically designed for CentOS).
>    Does anyone know what is causing this/how to fix it?

Have you checked "xm info" to see the Xen hypervisor memory info?
Maybe it's just the dom0 linux kernel that doesn't have more than 32G,
and the Xen hypervisor sees all of it.

Anyways, you should always limit dom0 memory to 512 MB or 1 GB to disable
Xen memory ballooning.

If you boot dom0 with all the (32G of) memory visible to it, and then
create VMs, and thus balloon dom0 down to much smaller amount of memory,
you'll get all kinds of problems when the dom0 linux memory suddenly is
much smaller than it was during boot time.

You should always limit and dedicate some amount of memory to dom0.
Use dom0_mem=1G option for xen.gz in grub.conf.

-- Pasi

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