主机组（host aggregate）
主机组面向的是管理员; Available Zone面向的是所有用户
Available Zone下包含若干主机组。
一个主机可以存在多个主机组内 一个主机组可以包含多个key-value对 同一个key-value对可以被分配到多个主机组上
openstack官网（http://docs.openstack.org/trunk/config-reference/content/section_compute-scheduler.html） 关于nova schedule的说明中有个关于主机组的例子很形象：
Example: Specify compute hosts with SSDs This example configures the Compute service to enable users to request nodes that have solid-state drives (SSDs). You create a fast-io host aggregate in the nova availability zone and you add the ssd=true key-value pair to the aggregate. Then, you add the node1, and node2 compute nodes to it.
$ nova aggregate-create fast-io nova
+----+---------+-------------------+-------+----------+
| Id | Name | Availability Zone | Hosts | Metadata |
+----+---------+-------------------+-------+----------+
| 1 | fast-io | nova | | |
+----+---------+-------------------+-------+----------+
$ nova aggregate-set-metadata 1 ssd=true
+----+---------+-------------------+-------+-------------------+
| Id | Name | Availability Zone | Hosts | Metadata |
+----+---------+-------------------+-------+-------------------+
| 1 | fast-io | nova | [] | {u'ssd': u'true'} |
+----+---------+-------------------+-------+-------------------+
$ nova aggregate-add-host 1 node1
+----+---------+-------------------+-----------+--------------------+
| Id | Name | Availability Zone | Hosts | Metadata |
+----+---------+-------------------+------------+-------------------+
| 1 | fast-io | nova | [u'node1']| {u'ssd': u'true'} |
+----+---------+-------------------+------------+-------------------+
$ nova aggregate-add-host 1 node2
+----+---------+-------------------+---------------------+-------------------+
| Id | Name | Availability Zone | Hosts | Metadata |
+----+---------+-------------------+----------------------+-------------------+
| 1 | fast-io | nova | [u'node1', u'node2']| {u'ssd': u'true'} |
+----+---------+-------------------+----------------------+-------------------+
Use the nova flavor-create command to create the ssd.large flavor called with an ID of 6, 8 GB of RAM, 80 GB root disk, and four vCPUs.
$ nova flavor-create ssd.large 6 8192 80 4
+----+-----------+-----------+------+-----------+------+-------+-------------+-----------+-------------+
| ID | Name | Memory_MB | Disk | Ephemeral | Swap | VCPUs | RXTX_Factor | Is_Public | extra_specs |
+----+-----------+-----------+------+-----------+------+-------+-------------+-----------+-------------+
| 6 | ssd.large | 8192 | 80 | 0 | | 4 |1 | True | {} |
+----+-----------+-----------+------+-----------+------+-------+-------------+-----------+-------------+
Once the flavor is created, specify one or more key-value pairs that match the key-value pairs on the host aggregates. In this case, that is the ssd=true key-value pair. Setting a key-value pair on a flavor is done using the nova flavor-key command.$ nova flavor-key ssd.large set ssd=true
Once it is set, you should see the extra_specs property of the ssd.large flavor populated with a key of ssd and a corresponding value of true.
$ nova flavor-show ssd.large
+----------------------------+-------------------+
| Property | Value |
+----------------------------+-------------------+
| OS-FLV-DISABLED:disabled | False |
| OS-FLV-EXT-DATA:ephemeral | 0 |
| disk | 80 |
| extra_specs | {u'ssd': u'true'} |
| id | 6 |
| name | ssd.large |
| os-flavor-access:is_public | True |
| ram | 8192 |
| rxtx_factor | 1.0 |
| swap | |
| vcpus | 4 |
+----------------------------+-------------------+
Now, when a user requests an instance with the ssd.large flavor, the scheduler only considers hosts with the ssd=true key-value pair. In this example, these are node1 and node2.