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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
>