Bug 1427326

Deploying an overcloud with a large number nodes at once can result in a failed deployment (similar to the symptoms described in BZ 1410977 on OSP10). Rules of thumb have evolved to workaround this consisting of: 

1. Using scheduler hints to pin node placement instead of just letting tripleo's nova scheduler schedule the ironic nodes
2. Ensuring the undercloud.conf IP ranges are not too small
3. Deploying in batches 

One may deploy a larger overcloud when following the above, but waiting to come back and scale a subset of nodes at a time may not be the best deployer experience. 

As an alternative, could TripleO, be configured to do a deployment in batches so that a deployer could start a large deploy and it would batch the deployment for them? 

Could there be a new variable in TripleO called something like $batch_size which when given a deployment consisting of N total nodes stands up a smaller overcloud and then scale it slowly in batches of $batch_size so that the deployer does not need to worry about batching their deployment (but can change the batch size if they need to). 

Since the deployment is now managed by a Mistral workflow this type of batching for a long running task might seem like something that would fit Mistral well so I just wanted to track this idea in a BZ RFE and see if others vote it up.