On 20/02/15 11:48, Matthew Booth wrote:
> Gary Kotton came across a doozy of a bug recently:
> https://bugs.launchpad.net/nova/+bug/1419785
> In short, when you start a Nova compute, it will query the driver for
> instances and compare that against the expected host of the the instance
> according to the DB. If the driver is reporting an instance the DB
> thinks is on a different host, it assumes the instance was evacuated
> while Nova compute was down, and deletes it on the hypervisor. However,
> Gary found that you trigger this when starting up a backup HA node which
> has a different `host` config setting. i.e. You fail over, and the first
> thing it does is delete all your instances.
> Gary and I both agree on a couple of things:
> 1. Deleting all your instances is bad
> 2. HA nova compute is highly desirable for some drivers
> We disagree on the approach to fixing it, though. Gary posted this:
> https://review.openstack.org/#/c/154029/
> I've already outlined my objections to this approach elsewhere, but to
> summarise I think this fixes 1 symptom of a design problem, and leaves
> the rest untouched. If the value of nova compute's `host` changes, then
> the assumption that instances associated with that compute can be
> identified by the value of instance.host becomes invalid. This
> assumption is pervasive, so it breaks a lot of stuff. The worst one is
> _destroy_evacuated_instances(), which Gary found, but if you scan
> nova/compute/manager for the string 'self.host' you'll find lots of
> them. For example, all the periodic tasks are broken, including image
> cache management, and the state of ResourceTracker will be unusual.
> Worse, whenever a new instance is created it will have a different value
> of instance.host, so instances running on a single hypervisor will
> become partitioned based on which nova compute was used to create them.
> In short, the system may appear to function superficially, but it's
> unsupportable.
> I had an alternative idea. The current assumption is that the `host`
> managing a single hypervisor never changes. If we break that assumption,
> we break Nova, so we could assert it at startup and refuse to start if
> it's violated. I posted this VMware-specific POC:
> https://review.openstack.org/#/c/154907/
> However, I think I've had a better idea. Nova creates ComputeNode
> objects for its current configuration at startup which, amongst other
> things, are a map of host:hypervisor_hostname. We could assert when
> creating a ComputeNode that hypervisor_hostname is not already
> associated with a different host, and refuse to start if it is. We would
> give an appropriate error message explaining that this is a
> misconfiguration. This would prevent the user from hitting any of the
> associated problems, including the deletion of all their instances.

I have posted a patch implementing the above for review here:


Matthew Booth
Red Hat Engineering, Virtualisation Team

Phone: +442070094448 (UK)
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