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[Openstack] [nova] Nova-scheduler: when are filters applied?


On 08/30/2018 08:54 AM, Eugen Block wrote:
> Hi Jay,
>
>> You need to set your ram_allocation_ratio nova.CONF option to 1.0 if you're
>> running into OOM issues. This will prevent overcommit of memory on your
>> compute nodes.
>
> I understand that, the overcommitment works quite well most of the time.
>
> It just has been an issue twice when I booted an instance that had been shutdown
> a while ago. In the meantime there were new instances created on that
> hypervisor, and this old instance caused the OOM.
>
> I would expect that with a ratio of 1.0 I would experience the same issue,
> wouldn't I? As far as I understand the scheduler only checks at instance
> creation, not when booting existing instances. Is that a correct assumption?

The system keeps track of how much memory is available and how much has been 
assigned to instances on each compute node.  With a ratio of 1.0 it shouldn't 
let you consume more RAM than is available even if the instances have been shut 
down.

Chris