97 lines
6.3 KiB
XML
97 lines
6.3 KiB
XML
<!DOCTYPE section [
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<!ENTITY % openstack SYSTEM "../../common/entities/openstack.ent">
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%openstack;
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]><section xml:id="configuring-compute-service-groups"
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xmlns="http://docbook.org/ns/docbook"
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xmlns:xlink="http://www.w3.org/1999/xlink"
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xmlns:xi="http://www.w3.org/2001/XInclude"
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version="5.0">
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<title>Configure Compute service groups</title>
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<para>To effectively manage and utilize compute nodes, the Compute service must know their
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statuses. For example, when a user launches a new VM, the Compute scheduler sends the
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request to a live node; the Compute service queries the ServiceGroup API to get information
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about whether a node is alive.</para>
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<para>When a compute worker (running the <systemitem class="service">nova-compute</systemitem>
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daemon) starts, it calls the <systemitem>join</systemitem> API to join the compute group.
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Any interested service (for example, the scheduler) can query the group's membership and the
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status of its nodes. Internally, the <systemitem>ServiceGroup</systemitem> client driver
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automatically updates the compute worker status.</para>
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<para>The database, ZooKeeper, and Memcache drivers are available.</para>
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<section xml:id="database-servicegroup-driver">
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<title>Database ServiceGroup driver</title>
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<para>By default, Compute uses the database driver to track node liveness. In a compute worker,
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this driver periodically sends a <command>db update</command> command to the database,
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saying <quote>I'm OK</quote> with a timestamp. Compute uses a pre-defined timeout
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(<literal>service_down_time</literal>) to determine whether a node is dead.</para>
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<para>The driver has limitations, which can be an issue depending on your setup. The more compute
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worker nodes that you have, the more pressure you put on the database. By default, the
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timeout is 60 seconds so it might take some time to detect node failures. You could
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reduce the timeout value, but you must also make the database update more frequently,
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which again increases the database workload.</para>
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<para>The database contains data that is both transient (whether the node is alive) and persistent
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(for example, entries for VM owners). With the ServiceGroup abstraction, Compute can treat
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each type separately.</para>
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</section>
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<section xml:id="zookeeper-servicegroup-driver">
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<title>ZooKeeper ServiceGroup driver</title>
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<para>The ZooKeeper ServiceGroup driver works by using ZooKeeper
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ephemeral nodes. ZooKeeper, in contrast to databases, is a
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distributed system. Its load is divided among several servers.
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At a compute worker node, after establishing a ZooKeeper session,
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the driver creates an ephemeral znode in the group directory. Ephemeral
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znodes have the same lifespan as the session. If the worker node
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or the <systemitem class="service">nova-compute</systemitem> daemon crashes, or a network
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partition is in place between the worker and the ZooKeeper server quorums,
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the ephemeral znodes are removed automatically. The driver
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gets the group membership by running the <command>ls</command> command in the group directory.</para>
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<para>To use the ZooKeeper driver, you must install ZooKeeper servers and client libraries.
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Setting up ZooKeeper servers is outside the scope of this guide (for more information,
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see <link xlink:href="http://zookeeper.apache.org/"
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>Apache Zookeeper</link>).</para>
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<para>To use ZooKeeper, you must install client-side Python libraries on every nova node:
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<literal>python-zookeeper</literal> – the official Zookeeper Python binding
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and <literal>evzookeeper</literal> – the library to make the binding work with the
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eventlet threading model.</para>
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<para>The following example assumes the ZooKeeper server addresses and ports are
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<literal>192.168.2.1:2181</literal>, <literal>192.168.2.2:2181</literal>, and
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<literal>192.168.2.3:2181</literal>.</para>
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<para>The following values in the <filename>/etc/nova/nova.conf</filename> file (on every
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node) are required for the <systemitem>ZooKeeper</systemitem> driver:</para>
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<programlisting language="ini"># Driver for the ServiceGroup service
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servicegroup_driver="zk"
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[zookeeper]
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address="192.168.2.1:2181,192.168.2.2:2181,192.168.2.3:2181"</programlisting>
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<para>To customize the Compute Service groups, use the following configuration option
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settings:</para>
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<xi:include href="../../common/tables/nova-zookeeper.xml"/>
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</section>
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<section xml:id="memcache-servicegroup-driver">
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<title>Memcache ServiceGroup driver</title>
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<para>The <systemitem>memcache</systemitem> ServiceGroup driver uses memcached, which is a
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distributed memory object caching system that is often used to increase site
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performance. For more details, see <link xlink:href="http://memcached.org/"
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>memcached.org</link>.</para>
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<para>To use the <systemitem>memcache</systemitem> driver, you must install
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<systemitem>memcached</systemitem>. However, because
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<systemitem>memcached</systemitem> is often used for both OpenStack Object Storage
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and OpenStack dashboard, it might already be installed. If
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<systemitem>memcached</systemitem> is not installed, refer to the <link
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xlink:href="http://docs.openstack.org/icehouse"
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><citetitle>OpenStack Installation Guide</citetitle></link> for more
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information.</para>
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<para>The following values in the <filename>/etc/nova/nova.conf</filename> file (on every
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node) are required for the <systemitem>memcache</systemitem> driver:</para>
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<programlisting language="ini"># Driver for the ServiceGroup service
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servicegroup_driver="mc"
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# Memcached servers. Use either a list of memcached servers to use for caching (list value),
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# or "<None>" for in-process caching (default).
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memcached_servers=<None>
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# Timeout; maximum time since last check-in for up service (integer value).
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# Helps to define whether a node is dead
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service_down_time=60</programlisting>
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</section>
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</section>
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