[openstack-dev] [Nova] Reconciling flavors and block device mappings

Jay Pipes jaypipes at gmail.com
Mon Aug 29 11:25:08 UTC 2016

On 08/26/2016 09:20 AM, Ed Leafe wrote:
> On Aug 25, 2016, at 3:19 PM, Andrew Laski <andrew at lascii.com> wrote:
>> One other thing to note is that while a flavor constrains how much local
>> disk is used it does not constrain volume size at all. So a user can
>> specify an ephemeral/swap disk <= to what the flavor provides but can
>> have an arbitrary sized root disk if it's a remote volume.
> This kind of goes to the heart of the argument against flavors being the sole source of truth for a request. As cloud evolves, we keep packing more and more stuff into a concept that was originally meant to only divide up resources that came bundled together (CPU, RAM, and local disk). This hasn’t been a good solution for years, and the sooner we start accepting that a request can be much more complex than a flavor can adequately express, the better.
> If we have decided that remote volumes are a good thing (I don’t think there’s any argument there), then we should treat that part of the request as being as fundamental as a flavor. We need to make the scheduler smarter so that it doesn’t rely on flavor as being the only source of truth.
> The first step to improving Nova is admitting we have a problem. :)

FWIW, I agree with you on the above. The issue I had with the proposed 
patches was that they would essentially be a hack for a short period of 
time until the resource providers work standardized the way that DISK_GB 
resources were tracked -- including for providers of shared disk storage.

I've long felt that flavors as a concept should be, as Chris so adeptly 
wrote, "UI furniture" and should be decomposed into their requisite 
lists of resource amounts, required traits and preferred traits and that 
those decomposed parts are what should be passed to the Compute API, not 
a flavor ID.

But again, we're actively changing all this code in the resource 
providers and qualitative traits patches so I warned about adding more 
code that was essentially just a short-lived hack. I'd be OK adding the 
hack code if there were some big bright WARNINGs placed in there that 
likely the code would be removed in Ocata.


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