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Le 23/09/2015 05:24, Zhenyu Zheng a écrit :
> Hi, all
>
> I have a question about availability zones when performing live-migration.
>
> Currently, when performing live-migration the AZ of the instance 
> didn't update. In usecase like this:
> Instance_1 is in host1 which is in az1, we live-migrate it to host2 
> (provide host2 in API request) which is in az2. The operation will 
> secusess but the availability zone data stored in instance1 is still 
> az1, this may cause inconsistency with the az data stored in instance 
> db and the actual az. I think update the az information in instance 
> using the host az can solve this.
>

Well, no. Instance.AZ is only the reflect of what the user asked, not 
what the current AZ is from the host the instance belongs to. In other 
words, instance.az is set once forever by taking the --az hint from the 
API request and persisting it in DB.

That means that if you want to create a new VM without explicitly 
specifying one AZ in the CLI, it will take the default value of 
CONF.default_schedule_az which is None (unless you modify that flag).

Consequently, when it will go to the scheduler, the AZFilter will not 
check the related AZs from any host because you didn't asked for an AZ. 
That means that the instance is considered "AZ-free".

Now, when live-migrating, *if you specify a destination*, you totally 
bypass the scheduler and thus the AZFilter. By doing that, you can put 
your instance to another host without really checking the AZ.

That said, if you *don't specify a destination*, then the scheduler will 
be called and will enforce the instance.az field with regards to the 
host AZ. That should still work (again, depending on whether you 
explicitly set an AZ at the boot time)

To be clear, there is no reason of updating that instance AZ field. We 
can tho consider it's a new "request"' field and could be potentially 
moved to the RequestSpec object, but for the moment, this is a bit too 
early since we don't really use that new RequestSpec object yet.



> Also, I have heard from my collegue that in the future we are planning 
> to use host az information for instances. I couldn't find informations 
> about this, could anyone provide me some information about it if thats 
> true?
>

See my point above, I'd rather prefer to fix how live-migrations check 
the scheduler (and not bypass it when specifying a destination) and 
possibly move the instance AZ field to the RequestSpec object once that 
object is persisted, but I don't think we should check the host instead 
of the instance in the AZFilter.


I assume all of that can be very confusing and mostly tribal knowledge, 
that's why we need to document that properly and one first shot is 
https://review.openstack.org/#/c/223802/

-Sylvain

> Thanks,
>
> Best Regards,
>
> Zheng
>
>
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