[Openstack-operators] Overselling/Oversubscribing RAM
Diego Parrilla Santamaría
diego.parrilla.santamaria at gmail.com
Mon Sep 24 16:12:40 UTC 2012
Yes, overcommiting or not overcommiting RAM: that is the question ;-)
The only reason for not recommending memory overcommitment is because it's
hard to define a good Service Level Agreement.
KVM does allow memory overcommitment and it works. We can guarantee it. KSM
works (http://www.linux-kvm.org/page/KSM). But it's difficult to predict
the performance impact of this technique. Hence, we can find situations
where a Service Provider using StackOps Enterprise Edition (Openstack Nova
on steroids in the end) can complain of unexpected bad performance because
a customer uses big Windows VMs in a physical server mixed with FreeBSD and
Linux, for example.
So, if you can live without strict SLAs, no problem. But if your business
relies on SLAs signed with end customers, you should better avoid memory
overcommitment. RAM is not so expensive and may be you can overcommit
storage instead, for example.
P.D.: What I don't recommend in any way is overselling resources.
Overcommitting must be a very controlled technique. I consider overselling
*www.stackops.com | * diego.parrilla at stackops.com** | +34 649 94 43 29 |
On Mon, Sep 24, 2012 at 5:40 PM, Joe Topjian <joe.topjian at cybera.ca> wrote:
> Hi Diego,
> Thank you for the link.
> Can you comment further on why you do not recommend overcommitting RAM?
> By default, OpenStack overcommits RAM by a ratio of 1.5. Do you change
> this setting to 1?
> What kind of side effects have you seen from overcommitting?
> On Mon, Sep 24, 2012 at 1:57 AM, Diego Parrilla Santamaría <
> diego.parrilla.santamaria at gmail.com> wrote:
>> Hi all,
>> We wrote several months ago a quick overview about how to oversubscribe
>> with StackOps Enterprise Edition. Surprisingly it was a presentation our
>> customer they love :-o
>> Here goes the presentation in slideshare:
>> Diego Parrilla
>> *www.stackops.com | * diego.parrilla at stackops.com** | +34 649 94 43 29 |
>> * <http://www.stackops.com/>
>> On Fri, Sep 21, 2012 at 4:39 PM, 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?
>>> OpenStack-operators mailing list
>>> OpenStack-operators at lists.openstack.org
>> OpenStack-operators mailing list
>> OpenStack-operators at lists.openstack.org
> Joe Topjian
> Systems Administrator
> Cybera Inc.
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