[openstack-dev] [nova] Question about boot-from-volume instance and flavor

Jay Pipes jaypipes at gmail.com
Tue Mar 3 20:45:06 UTC 2015

On 03/03/2015 01:10 AM, Rui Chen wrote:
> Hi all,
> When we boot instance from volume, we find some ambiguous description
> about flavor root_gb in operations guide,
> http://docs.openstack.org/openstack-ops/content/flavors.html
> /Virtual root disk size in gigabytes. This is an ephemeral disk the base
> image is copied into. You don't use it when you boot from a persistent
> volume. /
> /The "0" size is a special case that uses the native base image size as
> the size of the ephemeral root volume./
> /
> /
> 'You don't use it(root_gb) when you boot from a persistent volume.'
> It means that we need to set the root_gb to 0 or not? I don't know.

Hi Rui, I agree the documentation -- and frankly, the code in Nova -- is 
confusing around this area.

> But I find out that the root_gb will been added into local_gb_used of
> compute_node so that it will impact the next scheduling. Think about a
> use case, the local_gb of compute_node is 10, boot instance from volume
> with the root_gb=5 flavor, in this case, I can only boot 2
> boot-from-volume instances on the compute_nodes, although these
> instances don't use the local disk of compute_nodes.
> I find a patch that try to fix this issue,
> https://review.openstack.org/#/c/136284/
> I want to know that which solution is better for you?
> Solution #1: boot instance from volume with the root_gb=0 flavor.
> Solution #2: add some special logic in order to correct the disk usage,
> like patch #136284

Solution #2 is a better idea, IMO. There should not be any "magic" 
setting for root_gb that needs to be interpreted both by the user and 
the Nova code base.

The issue with the 136284 patch is that it is trying to address the 
problem in the wrong place, IMHO.


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