[Openstack-operators] Overselling/Oversubscribing RAM

Joe Topjian joe.topjian at cybera.ca
Sun Sep 23 01:04:18 UTC 2012

Hi Samuel,

I like the formula given here:


(0.5 * RAM) + (overcommit ratio * RAM) = Recommended swap size

This will ensure you never run out of swap space when overcommitting RAM --
which is imperative as described by the big red WARNING message in the
link.  :)

As far as I am aware, the other methods that KVM uses for memory management
(KSM and ballooning) are enabled and managed by default and shouldn't have
to be tweaked -- but someone please correct me if I am wrong.


On Fri, Sep 21, 2012 at 8:39 AM, Samuel Winchenbach <swinchen at gmail.com>wrote:

> Hi All,
> What are some "best practices" when it comes to overselling RAM in a
> KVM based openstack setup?  For example:
> I have 4 nodes with 16 cores & 128 GB RAM each.  If on one of those
> nodes I oversell ram 2:1 (256 GB RAM) and the VMs actually use that
> much does it just start swapping out pages to disk?
> How do all of you handle overselling RAM?
> Thanks,
> Sam
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Joe Topjian
Systems Administrator
Cybera Inc.


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