[Openstack-operators] over commit ratios

George Shuklin george.shuklin at gmail.com
Wed Apr 22 12:09:47 UTC 2015

Yes, it really depends on the used backing technique. We using SSDs and 
raw images, so IO is not an issue.

But memory is more important: if you lack IO capability you left with 
slow guests. If you lack memory you left with dead guests (hello, OOM 

BTW: Swap is needed not to swapin/swapout, but to relief memory 
pressure. With properly configured memory swin/swout  should be less 
than 2-3.

On 04/22/2015 09:49 AM, Tim Bell wrote:
> I'd also keep an eye on local I/O... we've found this to be the resource which can cause the worst noisy neighbours. Swapping makes this worse.
> Tim
>> -----Original Message-----
>> From: George Shuklin [mailto:george.shuklin at gmail.com]
>> Sent: 21 April 2015 23:55
>> To: openstack-operators at lists.openstack.org
>> Subject: Re: [Openstack-operators] over commit ratios
>> It's very depend on production type.
>> If you can control guests and predict their memory consumption, use it as base
>> for ratio.
>> If you can't (typical for public clouds) - use 1 or smaller with
>> reserved_host_memory_mb in nova.conf.
>> And one more: some swap sapce is really necessary. Add at least twice of
>> reserved_host_memory_mb - it really improves performance and prevents
>> strange OOMs in the situation of very large host with very small dom0 footprint.
>> On 04/21/2015 10:59 PM, Caius Howcroft wrote:
>>> Just a general question: what kind of over commit ratios do people
>>> normally run in production with?
>>> We currently run 2 for cpu and 1 for memory (with some held back for
>>> OS/ceph)
>>> i.e.:
>>> default['bcpc']['nova']['ram_allocation_ratio'] = 1.0
>>> default['bcpc']['nova']['reserved_host_memory_mb'] = 1024 # often
>>> larger default['bcpc']['nova']['cpu_allocation_ratio'] = 2.0
>>> Caius
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