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| <p class="title is-size-2 is-centered">Usage Scenario 功能</p> |
| </div> |
| </section> |
| <section class="section" style="padding-top: 2rem;"> |
| <div class="content"> |
| <blockquote> |
| <p>从 1.8.1 版本开始，Pegasus 支持了 Usage Scenario 功能。</p> |
| </blockquote> |
| |
| <h1 id="原理">原理</h1> |
| |
| <p>Usage Scenario 功能，是指对 Pegasus 表指定 <em>使用场景</em>。针对不同的场景，通过优化底层 RocksDB 的配置，以获得更好的读写性能。</p> |
| |
| <p>RocksDB 采用了 LSM tree 存储架构，其中的 <a href="https://github.com/facebook/rocksdb/wiki/Compaction">Compaction</a> 会较大地影响读写性能，Pegasus 采用了 Classic Leveled 算法，他的 Compaction 原理参考 <a href="https://github.com/facebook/rocksdb/wiki/Leveled-Compaction">Leveled-Compaction</a>。</p> |
| |
| <p>RocksDB 是一个可配置性很强的引擎，各种 flush 操作和 compaction 操作都可以通过配置调节，并且有部分配置是可以运行时修改的。这里给出几个比较关键的配置：</p> |
| <blockquote> |
| <p>（配置说明源自 RocksDB 源码）</p> |
| <ul> |
| <li>write_buffer_size: Amount of data to build up in memory before converting to a sorted on-disk file.</li> |
| <li>level0_file_num_compaction_trigger: Number of files to trigger level-0 compaction. A value <0 means that level-0 compaction will not be triggered by number of files at all.</li> |
| <li>level0_slowdown_writes_trigger：Soft limit on number of level-0 files. We start slowing down writes at this point. A value <0 means that no writing slow down will be triggered by number of files in level-0.</li> |
| <li>level0_stop_writes_trigger: Maximum number of level-0 files. We stop writes at this point.</li> |
| <li>max_bytes_for_level_base: Control maximum total data size for a level. max_bytes_for_level_base is the max total for level-1.</li> |
| <li>max_bytes_for_level_multiplier: Maximum number of bytes for level L can be calculated as (max_bytes_for_level_base) * (max_bytes_for_level_multiplier ^ (L-1)).</li> |
| </ul> |
| </blockquote> |
| |
| <p>Pegasus 在提供读写服务时，需要考虑这些因素：</p> |
| <ul> |
| <li>写操作越快，memtable 就会更快写满，就会更快地 flush memtable 产生 level-0 上新的 sstable 文件</li> |
| <li>随着 level-0 上 sstable 文件的累积，就会触发 compaction 操作，并从低层向高层逐层蔓延</li> |
| <li>Compaction 操作越多，耗费的 CPU 和磁盘 IO 负载越高，从而影响读写操作的性能</li> |
| <li>如果 level-0 到 level-1 的 compaction 操作速度低于数据写入的速度，level-0 的文件数就会累积得越多，最终达到 <code class="language-plaintext highlighter-rouge">level0_slowdown_writes_trigger</code> 阈值，使写操作的延迟陡增，甚至进一步达到 <code class="language-plaintext highlighter-rouge">level0_stop_writes_trigger</code> 阈值，使写操作失败，影响系统的稳定性和服务的可用性</li> |
| <li>高吞吐且低延迟的读写需求很难同时得到满足，二者需要权衡。 |
| <ul> |
| <li>Compaction 操作进行得越快，level-0 累积得文件数越少，读操作需要读取的文件个数就越少，读性能就越高</li> |
| <li>但是 compaction 越快，带来的写放大就越大，CPU 和磁盘 IO 负载就越高，也会影响读写性能</li> |
| </ul> |
| </li> |
| </ul> |
| |
| <p>所幸的是，RocksDB 针对这个问题也给出了一些解决方案，例如在 <a href="https://github.com/facebook/rocksdb/wiki/RocksDB-FAQ">RocksDB-FAQ</a> 中给出的方案：</p> |
| |
| <blockquote> |
| <p>Q: What’s the fastest way to load data into RocksDB?</p> |
| |
| <p>A: A fast way to direct insert data to the DB:</p> |
| |
| <ol> |
| <li>using single writer thread and insert in sorted order</li> |
| <li>batch hundreds of keys into one write batch</li> |
| <li>use vector memtable</li> |
| <li>make sure options.max_background_flushes is at least 4</li> |
| <li>before inserting the data, disable automatic compaction, set options.level0_file_num_compaction_trigger, options.level0_slowdown_writes_trigger and options.level0_stop_writes_trigger to very large value. After inserting all the data, issue a manual compaction.</li> |
| </ol> |
| |
| <p>3-5 will be automatically done if you call Options::PrepareForBulkLoad() to your option</p> |
| </blockquote> |
| |
| <p>Pegasus 的解决方案是，针对不同应用场景，设置不同的 RocksDB 参数，调节 RocksDB 的行为，以提供更好的读写性能。具体来说：</p> |
| <ul> |
| <li>通过 <a href="table-env">Table 环境变量</a> 设置 <code class="language-plaintext highlighter-rouge">rocksdb.usage_scenario</code> 来指定对应的应用场景</li> |
| <li>各表的 replica 在检测到该环境变量发生变化时，就会根据业务场景修改 RocksDB 的配置参数。 |
| <blockquote> |
| <p>具体设置了哪些参数，可参考 <a href="https://github.com/apache/incubator-pegasus/blob/master/src/server/pegasus_server_impl.cpp">src/server/pegasus_server_impl.cpp</a> 中的 <code class="language-plaintext highlighter-rouge">set_usage_scenario()</code> 方法。</p> |
| </blockquote> |
| </li> |
| </ul> |
| |
| <h1 id="支持场景">支持场景</h1> |
| |
| <p>目前 Pegasus 支持三种场景：</p> |
| <ul> |
| <li>normal：正常场景，读写兼顾。这也是表的默认场景，该场景不会对写进行特别的优化，适合大部分读写均衡的应用</li> |
| <li>prefer_write：写多读少的场景。主要是增大 <code class="language-plaintext highlighter-rouge">write_buffer_size</code> 和 <code class="language-plaintext highlighter-rouge">level0_file_num_compaction_trigger</code> 以降低 memtable 的 flush 操作和 level-0 到 level-1 的 compaction 操作的速度</li> |
| <li>bulk_load：批量导入数据的场景（注意这不是 <a href="https://pegasus.apache.org/zh/2020/02/18/bulk-load-design.html">bulk-load</a>）。使用上面 RocksDB-FAQ 中提到的优化，禁用 compaction 操作。此时，所有新写入的数据都会堆积在 level-0 层，对读不友好。因此，<code class="language-plaintext highlighter-rouge">bulk_load</code> 场景通常与 <a href="manual-compact">Manual Compact 功能</a> 结合使用，在数据导入完成后，进行一次 Manual Compact，以快速进行垃圾回收、全局排序来提升读性能，然后恢复为 normal 模式。典型的批量数据导入流程： |
| <ul> |
| <li>设置表的 <code class="language-plaintext highlighter-rouge">rocksdb.usage_scenario</code> 模式为 <code class="language-plaintext highlighter-rouge">bulk_load</code></li> |
| <li>导入数据：在 <code class="language-plaintext highlighter-rouge">bulk_load</code> 场景下数据写入 TPS 会更高，流量更稳定</li> |
| <li>执行 Manual Compact：该过程会消耗大量的 CPU 和磁盘 IO 资源，可能对集群读写性能有影响</li> |
| <li>恢复表的 <code class="language-plaintext highlighter-rouge">rocksdb.usage_scenario</code> 模式为 <code class="language-plaintext highlighter-rouge">normal</code></li> |
| </ul> |
| </li> |
| </ul> |
| |
| <h1 id="如何使用">如何使用</h1> |
| |
| <h2 id="通过-shell-工具设置">通过 shell 工具设置</h2> |
| |
| <p>通过 shell 工具的 <a href="/zh/docs/tools/shell/#set_app_envs">set_app_envs</a> 命令来设置，例如设置 temp 表为 bulk_load 模式：</p> |
| <div class="language-plaintext highlighter-rouge"><div class="highlight"><pre class="highlight"><code>>>> use temp |
| >>> set_app_envs rocksdb.usage_scenario bulk_load |
| </code></pre></div></div> |
| |
| <blockquote> |
| <p>Table 的环境变量不会立即生效，大约需要等几十秒（取决于配置项 <code class="language-plaintext highlighter-rouge">[replication]config_sync_interval_ms</code>）后才能在所有 replica 上生效。</p> |
| </blockquote> |
| |
| <h2 id="通过辅助脚本设置">通过辅助脚本设置</h2> |
| |
| <p>Pegasus 提供了一个辅助脚本 <a href="https://github.com/apache/incubator-pegasus/blob/master/scripts/pegasus_set_usage_scenario.sh">scripts/pegasus_set_usage_scenario.sh</a> 来方便地设置环境变量，用法：</p> |
| <div class="language-plaintext highlighter-rouge"><div class="highlight"><pre class="highlight"><code>$ ./scripts/pegasus_set_usage_scenario.sh |
| This tool is for set usage scenario of specified table(app). |
| USAGE: ./scripts/pegasus_set_usage_scenario.sh <cluster-meta-list> <app-name> <normal|prefer_write|bulk_load> |
| </code></pre></div></div> |
| |
| <p>该脚本会调用 shell 工具中设置 Table 环境变量的命令，然后检测是否在所有 replica 上都已经生效，只有所有 replica 上都生效了才算执行完成。</p> |
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