[Rdo-list] New deployment model for HA compute nodes - now with automated recovery of VMs

[Andrew Beekhof]

> On 9 Apr 2015, at 6:52 am, Arkady_Kanevsky at DELL.com wrote:
> 
> Dell - Internal Use - Confidential 
> 
> Does that work with HA controller cluster where pacemaker non-remote runs?

I'm not sure I understand the question.
The compute and control nodes are all part of a single cluster, its just that the compute nodes are not running a full stack.

Or do you mean, "could the same approach work for control nodes"?
For example, could this be used to manage more than 16 swift ACO nodes... 

Short answer: yes
Longer answer: yes, but there would likely need additional integration work required so don't expect it in a hurry

That specific case is on my mental list of options to explore in the future.

-- Andrew

> 
> -----Original Message-----
> From: rdo-list-bounces at redhat.com [mailto:rdo-list-bounces at redhat.com] On Behalf Of Andrew Beekhof
> Sent: Tuesday, April 07, 2015 9:13 PM
> To: rdo-list at redhat.com; rhos-pgm
> Cc: milind.manjrekar at redhat.com; Perry Myers; Marcos Garcia; Balaji Jayavelu
> Subject: [Rdo-list] New deployment model for HA compute nodes - now with automated recovery of VMs
> 
> Previously in order monitor the healthiness of compute nodes and the services running on them, we had to create single node clusters due to corosync's scaling limits.
> We can now announce a new deployment model that allows Pacemaker to continue this role, but presents a single coherent view of the entire deployment while allowing us to scale beyond corosync's limits.
> 
> Having this single administrative domain then allows us to do clever things like automated recovery of VMs running on a failed or failing compute node.
> 
> The main difference with the previous deployment mode is that services on the compute nodes are now managed and driven by the Pacemaker cluster on the control plane.
> The compute nodes do not become full members of the cluster and they no longer require the full cluster stack, instead they run pacemaker_remoted which acts as a conduit. 
> 
> Implementation Details:
> 
> - Pacemaker monitors the connection to pacemaker_remoted to verify that the node is reachable or not. 
>  Failure to talk to a node triggers recovery action.
> 
> - Pacemaker uses pacemaker_remoted to start compute node services in the same sequence as before (neutron-ovs-agent -> ceilometer-compute -> nova-compute).
> 
> - If a service fails to start, any services that depend on the FAILED service will not be started.
>  This avoids the issue of adding a broken node (back) to the pool.
> 
> - If a service fails to stop, the node where the service is running will be fenced. 
>  This is necessary to guarantee data integrity and a core HA concept (for the purposes of this particular discussion, please take this as a given).
> 
> - If a service's health check fails, the resource (and anything that depends on it) will be stopped and then restarted.
>  Remember that failure to stop will trigger a fencing action.
> 
> - A successful restart of all the services can only potentially affect network connectivity of the instances for a short period of time.
> 
> With these capabilities in place, we can exploit Pacemaker's node monitoring and fencing capabilities to drive nova host-evacuate for the failed compute nodes and recover the VMs elsewhere.
> When a compute node fails, Pacemaker will:
> 
> 1. Execute 'nova service-disable'
> 2. fence (power off) the failed compute node 3. fence_compute off (waiting for nova to detect the compute node is gone) 4. fence_compute on (a no-op unless the host happens to be up already) 5. Execute 'nova service-enable' when the compute node returns
> 
> Technically steps 1 and 5 are optional and they are aimed to improve user experience by immediately excluding a failed host from nova scheduling. 
> The only benefit is a faster scheduling of VMs that happens during a failure (nova does not have to recognize a host is down, timeout and subsequently schedule the VM on another host).
> 
> Step 2 will make sure the host is completely powered off and nothing is running on the host.
> Optionally, you can have the failed host reboot which would potentially allow it to re-enter the pool.
> 
> We have an implementation for Step 3 but the ideal solution depends on extensions to the nova API.
> Currently fence_compute loops, waiting for nova to recognise that the failed host is down, before we make a host-evacuate call which triggers nova to restart the VMs on another host.
> The discussed nova API extensions will speed up recovery times by allowing fence_compute to proactively push that information into nova instead.
> 
> 
> To take advantage of the VM recovery features:
> 
> - VMs need to be running off a cinder volume or using shared ephemeral storage (like RBD or NFS)
> - If VM is not running using shared storage, recovery of the instance on a new compute node would need to revert to a previously stored snapshot/image in Glance (potentially losing state, but in some cases that may not matter)
> - RHEL7.1+ required for infrastructure nodes (controllers and compute). Instance guests can run anything.
> - Compute nodes need to have a working fencing mechanism (IPMI, hardware watchdog, etc)
> 
> 
> Detailed instructions for deploying this new model are of course available on Github:
> 
>    https://github.com/beekhof/osp-ha-deploy/blob/master/ha-openstack.md#compute-node-implementation
> 
> It has been successfully deployed in our labs, but we'd really like to hear how it works for you in the field.
> Please contact me if you encounter any issues.
> 
> -- Andrew
> 
> _______________________________________________
> Rdo-list mailing list
> Rdo-list at redhat.com
> https://www.redhat.com/mailman/listinfo/rdo-list
> 
> To unsubscribe: rdo-list-unsubscribe at redhat.com