On 2/23/15, 2:05 PM, "Matthew Booth" <mbo...@redhat.com> wrote:

>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:

I have to look at what you have posted. I think that this topic is
something that we should speak about at the summit and this should fall
under some BP and well defined spec. I really would not like to see
existing installations being broken if and when this patch lands. It may
also affect Ironic as it works on the same model.

>Matthew Booth
>Red Hat Engineering, Virtualisation Team
>Phone: +442070094448 (UK)
>GPG ID:  D33C3490
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