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The option1 is a bad idea, IMO. Think about so many extra_specs in the
flavor today, you really don't want create another dozen of flavor
with root_gb=0 just for booting from volume.

For the billing concern, it really should be taken into account, but
it's not the business of OpenStack, IMHO.

I agree with Jay, option 2 will be good.

2015-03-05 13:58 GMT+08:00 Alex Xu <soulxu at gmail.com>:
>
>
> 2015-03-04 4:45 GMT+08:00 Jay Pipes <jaypipes at gmail.com>:
>>
>> 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.
>
>
> Emm.....I'm thinking of one case. There two flavors, one with root_gb=0,
> another one with root_gb=10. And user choice different flavor will pay
> different money.
> If user choice the flavor with root_gb=10, and then boot from volume...then
> user still need pay the extra money. That's good strange. Or say we should
> let the bill system to distinguish the instance is boot from volume or not.
> Or say we just tell user you make wrong choice... For this case, should we
> choice
> solution #1?
>
>>
>>
>> Best,
>> -jay
>>
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-- 
Regards!
-----------------------------------
Lingxian Kong