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<h1>高可用</h1>
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<p>Ozone 有两个leader节点（用于键管理的 <em>Ozone Manager</em> 和用于块空间管理的 <em>Storage Container Management</em> ）以及存储节点（数据节点）。数据是借助 RAFT 共识算法在数据节点之间复制。</p>
<p>为了避免任何单点故障，leader 节点也应该具备高可用的设置。</p>
<ol>
<li>Ozone Manager 的高可用是借助 RAFT 协议实现的；</li>
<li>Storage Container Manager 的高可用则 <a href="../../design/scmha.html">还在实现中</a> 。</li>
</ol>
<h2 id="ozone-manager-的高可用">Ozone Manager 的高可用</h2>
<p>一个 Ozone Manager 使用 <a href="https://github.com/facebook/rocksdb/">RocksDB</a> 在本地持久化元数据（卷、桶和键）。 Ozone Manager 的高可用版本在功能上完全一致，只是所有的数据都借助 RAFT 共识算法复制到 Ozone Manager 的 follower 实例上。</p>
<p><img src="HA-OM.png" alt="OM HA"></p>
<p>客户端连接到 Ozone Manager 上，而 Ozone Manager 负责处理请求并且安排复制。当请求复制到所有的 follower 上后，leader 就可以给客户端回包了。</p>
<h2 id="配置">配置</h2>
<p>可以在 <code>ozone-site.xml</code> 中配置以下设置来启用 Ozone Manager 的高可用模式：</p>
<div class="highlight"><pre style="color:#f8f8f2;background-color:#272822;-moz-tab-size:4;-o-tab-size:4;tab-size:4"><code class="language-XML" data-lang="XML"><span style="color:#f92672">&lt;property&gt;</span>
<span style="color:#f92672">&lt;name&gt;</span>ozone.om.ratis.enable<span style="color:#f92672">&lt;/name&gt;</span>
<span style="color:#f92672">&lt;value&gt;</span>true<span style="color:#f92672">&lt;/value&gt;</span>
<span style="color:#f92672">&lt;/property&gt;</span>
</code></pre></div><p>一个 Ozone 的配置（<code>ozone-site.xml</code>）支持多个 Ozone 高可用集群。为了支持在多个高可用集群之间进行选择，每个集群都需要一个逻辑名称，该逻辑名称可以解析为 Ozone Manager 的 IP 地址（和域名）。</p>
<p>该逻辑名称叫做 <code>serviceId</code>，可以在 <code>ozone-site.xml</code> 中进行配置：</p>
<pre><code>&lt;property&gt;
&lt;name&gt;ozone.om.service.ids&lt;/name&gt;
&lt;value&gt;cluster1,cluster2&lt;/value&gt;
&lt;/property&gt;
</code></pre><p>对于每个已定义的 <code>serviceId</code> ，还应为每个服务器定义一个逻辑配置名：</p>
<div class="highlight"><pre style="color:#f8f8f2;background-color:#272822;-moz-tab-size:4;-o-tab-size:4;tab-size:4"><code class="language-XML" data-lang="XML"><span style="color:#f92672">&lt;property&gt;</span>
<span style="color:#f92672">&lt;name&gt;</span>ozone.om.nodes.cluster1<span style="color:#f92672">&lt;/name&gt;</span>
<span style="color:#f92672">&lt;value&gt;</span>om1,om2,om3<span style="color:#f92672">&lt;/value&gt;</span>
<span style="color:#f92672">&lt;/property&gt;</span>
</code></pre></div><p>已定义的前缀可用于定义每个 OM 服务的地址：</p>
<div class="highlight"><pre style="color:#f8f8f2;background-color:#272822;-moz-tab-size:4;-o-tab-size:4;tab-size:4"><code class="language-XML" data-lang="XML"><span style="color:#f92672">&lt;property&gt;</span>
<span style="color:#f92672">&lt;name&gt;</span>ozone.om.address.cluster1.om1<span style="color:#f92672">&lt;/name&gt;</span>
<span style="color:#f92672">&lt;value&gt;</span>host1<span style="color:#f92672">&lt;/value&gt;</span>
<span style="color:#f92672">&lt;/property&gt;</span>
<span style="color:#f92672">&lt;property&gt;</span>
<span style="color:#f92672">&lt;name&gt;</span>ozone.om.address.cluster1.om2<span style="color:#f92672">&lt;/name&gt;</span>
<span style="color:#f92672">&lt;value&gt;</span>host2<span style="color:#f92672">&lt;/value&gt;</span>
<span style="color:#f92672">&lt;/property&gt;</span>
<span style="color:#f92672">&lt;property&gt;</span>
<span style="color:#f92672">&lt;name&gt;</span>ozone.om.address.cluster1.om3<span style="color:#f92672">&lt;/name&gt;</span>
<span style="color:#f92672">&lt;value&gt;</span>host3<span style="color:#f92672">&lt;/value&gt;</span>
<span style="color:#f92672">&lt;/property&gt;</span>
</code></pre></div><p>基于 <a href="../../zh/interface.html">客户端接口</a> ，定义好的 <code>serviceId</code> 就可用于替代单个 OM 主机。</p>
<p>例如，使用 <code>o3fs://</code></p>
<div class="highlight"><pre style="color:#f8f8f2;background-color:#272822;-moz-tab-size:4;-o-tab-size:4;tab-size:4"><code class="language-shell" data-lang="shell">hdfs dfs -ls o3fs://bucket.volume.cluster1/prefix/
</code></pre></div><p>或 <code>ofs://</code>:</p>
<div class="highlight"><pre style="color:#f8f8f2;background-color:#272822;-moz-tab-size:4;-o-tab-size:4;tab-size:4"><code class="language-shell" data-lang="shell">hdfs dfs -ls ofs://cluster1/volume/bucket/prefix/
</code></pre></div><h2 id="实现细节">实现细节</h2>
<p>只要请求成功持久化到了大多数节点的 RAFT 日志中，Raft 就可以保证请求的复制。 其中，为了基于 Ozone Manager 实现高吞吐量，即使请求仅持久化到了 RAFT 日志中，它也会立即返回响应。</p>
<p>RocksDB 由后台的批处理事务线程负责更新（这也就是所谓的&quot;双缓冲区&quot;，因为当一个缓冲区用于提交数据时，另一个缓冲区用于收集用于下一次提交的新请求）。这里为了使得当前所有数据对于后续请求都可见，即使后台线程还未完全将其写入，这些键数据也会被缓存在内存中。</p>
<p><img src="HA-OM-doublebuffer.png" alt="Double buffer"></p>
<p>尽管在单独的<a href="../../design/omha.html">设计文档</a>中介绍了这种方法的细节，但它仍算作是 OM 高可用的组成部分。</p>
<h2 id="参考文档">参考文档</h2>
<ul>
<li>查看 <a href="../../design/omha.html">该页面</a> 以获取详细设计文档；</li>
<li>Ozone 的分发包中的 <code>compose/ozone-om-ha</code> 目录下提供了一个配置 OM 高可用的示例，可以借助 <a href="../../start/runningviadocker.html">docker-compose</a> 进行测试。</li>
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