[nova] Strict isolation of group of hosts for image and flavor, modifying command 'nova-manage placement sync_aggregates'

Eric Fried openstack at fried.cc
Fri Jun 7 14:05:58 UTC 2019

>>> (a) Leave all traits alone. If they need to be removed, it would have to
>>> be manually via a separate step.
>>> (b) Support a new option so the caller can dictate whether the operation
>>> should remove the traits. (This is all-or-none.)
>>> (c) Define a "namespace" - a trait substring - and remove only traits in
>>> that namespace.
>> I'm going to -1 (b). It's too big a hammer, at too big a cost (including
>> API changes).
>>> If I’m not wrong, for last two approaches, we would need to change
>>> RestFul APIs.
>> No, (c) does not. By "define a namespace" I mean we would establish a
>> naming convention for traits to be used with this feature. For example:
> i personaly dislike c as it means we cannot use any standard traits in host
> aggrates.

Actually, it means we *can*, whereas (b) and (d) mean we *can't*. That's
the whole point. If we want to isolate our hyperthreading hosts, we put
them in an aggregate with HW_CPU_HYPERTHREADING on it. The sync here
should be a no-op because those hosts should already have
HW_CPU_HYPERTHREADING on them. And then if we decide to remove such a
host, or destroy the aggregate, or whatever, we *don't want*
HW_CPU_HYPERTHREADING to be removed from the providers, because they can
still do that.

(Unless you mean we can't make a standard trait that we can use for
isolation that gets (conditionally) removed in these scenarios? There's
nothing preventing us from creating a standard trait called

> there is also an option d. when you remove a trait form a host aggregate for each host in
> the aggregate check if that traits exists on another aggregate the host is a member of and remove
> it if not found on another aggregate.

Sorry I wasn't clear, (b) also does this ^ but with the condition that
also checks for the _AGGREGATE_ISOLATION_ infix.

> for c you still have to deal with the fact a host can be in multiple
host aggrates too by
> the way so jsut because a thread is namespace d and it is removed from
an aggrate does not
> mean its correcct to remove it from a host.

Right - in reality, there should be one algorithm, idempotent, to sync
host RP traits when anything happens to aggregates. It always goes out
and does the appropriate {set} math to decide which traits should exist
on which hosts and effects any necessary changes.

And yes, the performance will suck in a large deployment, because we
have to get all the compute RPs in all the aggregates (even the ones
with no trait metadata) to do that calculation. But aggregate operations
are fairly rare, aren't they?

Perhaps this is where we provide a nova-manage tool to do (b)'s sync
manually (which we'll surely have to do anyway as a "heal" or for
upgrades). So if you're not using the feature, you don't suffer the penalty.

> for example we talked about useing a hyperthreading trait in the cpu pinning spec which
> will not be managed by the compute driver. host aggages would be a convient way
> to be able to manage that trait if this was a generic feature.

Oh, okay, yeah, I don't accept this as a use case for this feature. It
will work, but we shouldn't recommend it precisely because it's
asymmetrical (you can't remove the trait by removing it from the
aggregate). There are other ways to add a random trait to all hosts in
an aggregate (for host in `get providers in aggregate`; do openstack
resource provider trait add ...; done).

But for the sake of discussion, what about:

(e) Fully manual. Aggregate operations never touch (add or remove)
traits on host RPs. You always have to do that manually. As noted above,
it's easy to do - and we could make it easier with a tiny wrapper that
takes an aggregate, a list of traits, and an --add/--remove command. So
initially, setting up aggregate isolation is a two-step process, and in
the future we can consider making new API/CLI affordance that combines
the steps.


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