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Found at: raymii.org:70/Openstack_Affinity_Groups-make-sure-instances-are-on-the-same-or-a-different-hypervisor-host.txt

This is a text-only version of the following page on https://raymii.org:
---
Title       : 	Openstack Affinity Groups, make sure instances are on the same or different compute hypervisor hosts
Author      : 	Remy van Elst
Date        : 	29-11-2014
URL         : 	https://raymii.org/s/articles/Openstack_Affinity_Groups-make-sure-instances-are-on-the-same-or-a-different-hypervisor-host.html
Format      : 	Markdown/HTML
---
This guide shows you how to use Openstack Affinity groups. Affinity or Anti-
Affinity groups allow you to make sure instances (VM/VPS) are on the same
hypervisor host or on a different one. There are cases when you want two
instances on different compute nodes, for example, when they are clustered
servers like a load balancer or a database master-master setup. All VM's in each
Affinity group are hosted in the same hypervisor, while no two VM's of a same
Anti-Affinity group are hosted in the same hypervisor.
Affinity is supported since Openstack IceHouse. You can only add an instance to
an affinity group when you create the instance with `nova boot`. You can not add
existing instances. You can however create an image of an instance and boot a
new server from that image, in the new affinity group.
You can see all my [Openstack related articles here][1]. For example, how to
build a [High Available cluster with Ansible and Openstack][2].

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### Create an Affinity Group
Make sure you have the Openstack Command Line Tools installed. Also make sure
you have a `computerc` file. See the first part of [this page][4] to find out
how to do that.
The command syntax to create an affinity group with the novaclient:
    
    
    nova server-group-create $group-name $policy
    
  * `$group-name` is a name you choose
  * `$policy` is either `affinity` or `anti-affinity`.
If you want all servers to run on the same hypervisor host, create a group with
an `affinity` policy.
If you want all servers to run on different hypervisor host, create a group with
an `anti-affinity` policy. This means that if you have 5 servers in this group,
they will be on 5 different hypervisor (compute) nodes, never will a VM be on a
host node where another VM from this affinity policy is.
You can see all available affinity groups with `server-group-list`:
    
    
    $ nova server-group-list
    +--------------------------------------+------+--------------------+---------+----------+
    | Id                                   | Name | Policies           | Members | Metadata |
    +--------------------------------------+------+--------------------+---------+----------+
    | 415eea22-4e37-44e0-a4c9-59c395688fc8 | Anti | [u'anti-affinity'] | []      | {}       |
    +--------------------------------------+------+--------------------+---------+----------+
    
You can delete a server group with `server-group-delete`. Deleting a server
group does not delete all the vm's in that group. The policy is removed from the
vm's, so they can be on the same node, or on different ones. Exclusivity is not
guaranteed anymore.
### Add instance to affinity group
You can only add a server to an affinity group when you create the instance. Not
afterwards. To add an instance to an affinity group, use the following extra
option when executing `nova boot`:
    
    
    --hint group=$affinity-group-uuid
    
Example commands:
    
    
    nova boot --image "CloudVPS Ubuntu 14.04" --hint group=415eea22-4e37-44e0-a4c9-59c395688fc8 --key-name $ssh_key --flavor "Standard 1" "Anti-Affinity 1-1"
    
    nova boot --image "CloudVPS Ubuntu 14.04" --hint group=415eea22-4e37-44e0-a4c9-59c395688fc8 --key-name $ssh_key --flavor "Standard 1" "Anti-Affinity 1-2"
    
    nova boot --image "CloudVPS Ubuntu 14.04" --hint group=415eea22-4e37-44e0-a4c9-59c395688fc8 --key-name $ssh_key --flavor "Standard 1" "Anti-Affinity 1-3"
    
    nova boot --image "CloudVPS Ubuntu 14.04" --hint group=415eea22-4e37-44e0-a4c9-59c395688fc8 --key-name $ssh_key --flavor "Standard 1" "Anti-Affinity 1-4"
    
These 4 servers will be on different hypervisor hosts. You can check this with
the `nova show` command:
    
    
    nova show $instance_uuid
    
Look for the `hostId` value:
    
    
    | hostId     | 4b499ffc320810e06891d15fca10a4ac76a50eae788d2f28a990fbbf     |
    
This is the hostId of the hosting compute hypervisor node. With an Anti-Affinity
policy, these are different. With an Affinity Policy, these are the same.
### No valid host was found
If you reveive the `No valid host was found` error during the `nova boot`
command, that means that there are no hosts that have the capacity to host that
VM and guarantee there will not be another VM from that anti-affinity group on
it (in the case of an anti affinity group). It can also mean that the hypervisor
has not enoigh free capacity (in the case of an affinity group).
This might mean that your Openstack provider has put restrictions on the filters
or is low on free capacity. Affinity policies are limited by the available
resources of the current hypervisor that hosts the group, while Anti-Affinity
policies are limited by the number of available hypervisors in the datacenter
(after filtering).
If you want your load balancers, app servers and database servers on different
compute nodes from each other (with the same role) then you can create 3 anti-
affinity groups, one for the load balancers, one for the app servers and one for
the database servers. Your app server might be on a host node with a load
balancer, but not with another app server.
### How to enable Affinity/Anti-Affinity (Admin Only)
Enabling Affinity and Anti-Afiinity is simply done by adding
`ServerGroupAffinityFilter` and `ServerGroupAntiAffinityFilter` into
`scheduler_default_filters`. By default these filters are already enabled:
    
    
    # nova.conf
    scheduler_default_filters = ServerGroupAffinityFilter,ServerGroupAntiAffinityFilter
    
To read more about the Openstack filtering options, see the official
documentation: .
   [1]: https://raymii.org/s/tags/openstack.html
   [2]: https://raymii.org/s/articles/Building_HA_Clusters_With_Ansible_and_Openstack.html
   [3]: https://www.digitalocean.com/?refcode=7435ae6b8212
   [4]: https://raymii.org/s/tutorials/FreeBSD_10.1-release_Openstack_Image.html
---
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