openstack中的Host Aggregates和Availability Zones
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Host Aggregates 的机制是建立于 Availability Zones 基础之上更进一步划分 computes 节点物理资源的一种机制。
什么是Region
Region更像是一个地理上的概念，每个region有自己独立的endpoint，regions之间完全隔离，但是多个regions之间共享同一个keystone和dashboard，region的设计更多侧重地理位置的概念，用户可以选择离自己更近的region来部署自己的服务。
什么是Availability Zones
Availability Zones 通常是对 computes 节点上的资源在小的区域内进行逻辑上的分组和隔离。例如在同一个数据中心，我们可以将 Availability Zones 规划到不同的机房，或者在同一机房的几个相邻的机架，从而保障如果某个 Availability Zone 的节点发生故障（如供电系统或网络），而不影响其他的 Availability Zones 上节点运行的虚拟机，通过这种划分来提高 OpenStack 的可用性。目前 OpenStack 默认的安装是把所有的 computes 节点划分到 nova 的 Availability Zone 上，但我们可以通过对 nova.conf 文件的配置来定义不同的 Availability zones。
什么是Host Aggregate
host aggregate是管理员用来根据硬件资源的某一属性来对硬件进行划分的功能，只对管理员可见，主要用来给nova-scheduler通过某一属性来进行instance的调度。其主要功能就是实现根据某一属性来划分物理机，比如按照地理位置，使用固态硬盘的机器，内存超过32G的机器，根据这些指标来构成一个host group。
Availability Zones和Host Aggregate的关系图
(图片来源于网络)
配置
/etc/nova/nova.conf
scheduler_default_filters=AggregateInstanceExtraSpecsFilter,其他filter
openstack官方使用host aggregate的例子
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 |
+----+-----------+-----------+------+-----------+------+-------+-------------+-----------+
| 6 | ssd.large | 8192 | 80 | 0 | | 4 | 1.0 | True |
+----+-----------+-----------+------+-----------+------+-------+-------------+-----------+
Once the flavor is created, specify one or more key-value pairs that match the key-value pairs on the host aggregates with scope aggregate_instance_extra_specs. In this case, that is the aggregate_instance_extra_specs: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 aggregate_instance_extra_specs: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'aggregate_instance_extra_specs: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.
nova实现 host aggregate的命令
# nova -h |grep aggregate
aggregate-add-host Add the host to the specified aggregate.
aggregate-create Create a new aggregate with the specified
aggregate-delete Delete the aggregate.
aggregate-details DEPRECATED, use aggregate-show instead.
aggregate-list Print a list of all aggregates.
aggregate-remove-host Remove the specified host from the specified
aggregate.
aggregate-set-metadata Update the metadata associated with the
aggregate.
aggregate-show Show details of the specified aggregate.
aggregate-update Update the aggregate's name and optionally
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