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

[Andrew Beekhof]

> On 11 Apr 2015, at 11:15 am, Tim Bell <Tim.Bell at cern.ch> wrote:
> 
> Andrew,
> 
>  
> 
> How much of this is RDO specific ?

Nothing at all provided you’re using Pacemaker.
Another cluster manager could be used but would require additional integration work by the interested party.

> It seems to be a generic approach similar to Russel Bryant’s blogs using pacemaker to get VM HA.

Russel worked with us to come up with that design, its the same thing :-)

> 
>  Can these approaches be converged to come up with a community supported approach for all OpenStack configurations ?
> 
>  
> 
> Tim
> 
>  
> 
> From: rdo-list-bounces at redhat.com [mailto:rdo-list-bounces at redhat.com] On Behalf Of Pedro Sousa
> Sent: 11 April 2015 01:32
> To: Andrew Beekhof
> Cc: milind.manjrekar at redhat.com; rdo-list at redhat.com; rhos-pgm; Perry Myers; Marcos Garcia; Balaji Jayavelu
> Subject: Re: [Rdo-list] New deployment model for HA compute nodes - now with automated recovery of VMs
> 
>  
> 
> Hi Andrew,
> 
>  
> 
> I've checked your git, great work, but I'm using native toois with keepalived approach, using mmonit utility to monitor the infrastructure without pacemaker/corosync.
> 
>  
> 
> I'm testing this approach to evacuate and disable a compute node, if something fails. What approach do you consider best, having in mind that a external monitoring tool like mmonit is not "cluster aware" and doesn't do things like fencingdead node like pacemaker does?
> 
>  
> 
> Thank you.
> 
>  
> 
> On Wed, Apr 8, 2015 at 3:12 AM, Andrew Beekhof <abeekhof at redhat.com> wrote:
> 
> 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
> 
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