How does the controller know which host to launch an instance on?

asked 2014-01-08 18:32:29 -0600

joethecoder gravatar image

updated 2014-01-22 15:12:36 -0600

Evgeny gravatar image


I am new to OpenStack. Trying to install a basic two node OpenStack on Ubuntu. One controller and one compute node. When I launch an instance from the command line on the controller after installing all the services, how does it know on which physical host the instance has to be launched? I am referring to OpenStack installation guide on Ubuntu. I do not see any command line parameter saying that this instance has to be launched on this host. Thanks!

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answered 2014-01-09 02:57:45 -0600

The "nova boot" command is simply a wrapper for the API. In other words, it can be used from any machine that has access to the environment. When the command is issued to the API, nova uses it's built in scheduler to determine the placement of the instance on a hypervisor. There are many different scheduler configurations, e.g. use the hypervisor with the fewest instances running, or find the hypervisor with the most memory free. Executing "nova boot" on a specific compute host will not guarantee its placement on that specific node.

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thanks for the clarification

joethecoder gravatar imagejoethecoder ( 2014-01-09 18:16:00 -0600 )edit

answered 2014-01-08 18:47:22 -0600

joethecoder gravatar image

I guess "nova boot" command is to be run on the compute node, and not the controller. I assumed that all the management commands would run on the controller. If that is correct, sorry for the noise.

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You need to run on the controller. Compute nodes are registered in the Controller. Once you start nova boot command request will go to compute nodes. If there are mutliple compute nodes which nodes the request is sent is based on many parameters. Hope this clarfies

dheeru gravatar imagedheeru ( 2014-01-08 19:51:33 -0600 )edit

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Asked: 2014-01-08 18:32:29 -0600

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Last updated: Jan 09 '14