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| <p class="title is-size-2 is-centered">Manual Compact功能</p> |
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| <section class="section" style="padding-top: 2rem;"> |
| <div class="content"> |
| <p>注：manual compact功能从v1.8.1版本开始支持。</p> |
| <h1 id="原理">原理</h1> |
| |
| <p>RocksDB除了根据需要自动触发compaction外，还能通过接口手动触发compaction，这个功能称之为<a href="https://github.com/facebook/rocksdb/wiki/Manual-Compaction">Manual Compaction</a>。其提供了<code class="language-plaintext highlighter-rouge">CompactRange()</code>接口，如下：</p> |
| <pre><code class="language-C++">// CompactRangeOptions is used by CompactRange() call. |
| struct CompactRangeOptions { |
| // If true, no other compaction will run at the same time as this |
| // manual compaction |
| bool exclusive_manual_compaction = true; |
| // If true, compacted files will be moved to the minimum level capable |
| // of holding the data or given level (specified non-negative target_level). |
| bool change_level = false; |
| // If change_level is true and target_level have non-negative value, compacted |
| // files will be moved to target_level. |
| int target_level = -1; |
| // Compaction outputs will be placed in options.db_paths[target_path_id]. |
| // Behavior is undefined if target_path_id is out of range. |
| uint32_t target_path_id = 0; |
| // By default level based compaction will only compact the bottommost level |
| // if there is a compaction filter |
| BottommostLevelCompaction bottommost_level_compaction = |
| BottommostLevelCompaction::kIfHaveCompactionFilter; |
| }; |
| |
| // Compact the underlying storage for the key range [*begin,*end]. |
| // The actual compaction interval might be superset of [*begin, *end]. |
| // In particular, deleted and overwritten versions are discarded, |
| // and the data is rearranged to reduce the cost of operations |
| // needed to access the data. This operation should typically only |
| // be invoked by users who understand the underlying implementation. |
| // |
| // begin==nullptr is treated as a key before all keys in the database. |
| // end==nullptr is treated as a key after all keys in the database. |
| // Therefore the following call will compact the entire database: |
| // db->CompactRange(options, nullptr, nullptr); |
| // Note that after the entire database is compacted, all data are pushed |
| // down to the last level containing any data. If the total data size after |
| // compaction is reduced, that level might not be appropriate for hosting all |
| // the files. In this case, client could set options.change_level to true, to |
| // move the files back to the minimum level capable of holding the data set |
| // or a given level (specified by non-negative options.target_level). |
| virtual Status CompactRange(const CompactRangeOptions& options, |
| ColumnFamilyHandle* column_family, |
| const Slice* begin, const Slice* end) = 0; |
| </code></pre> |
| |
| <p>对应地，Pegasus对该功能在上层进行了封装，提供了表级别的Manual Compact功能，其作用是：</p> |
| <ul> |
| <li>通过compaction去掉垃圾数据，减少数据量，降低文件层数，提升读操作的性能。</li> |
| <li>对最高层做compaction，可以清理掉Delete标记数据。</li> |
| <li>配合<a href="usage-scenario">Usage Scenario功能</a>中表的bulk_load模式，可以在灌数据完成后执行一次Manual Compact，去除垃圾数据，整理数据和文件夹结构，提升读性能。</li> |
| </ul> |
| |
| <p>实现方式：</p> |
| <ul> |
| <li>扩展RocksDB，在Manifest中记录上一次执行Manual Compact的时间，并提供<code class="language-plaintext highlighter-rouge">GetLastManualCompactFinishTime()</code>方法来获取该时间。</li> |
| <li>利用<a href="table-env">Table环境变量</a>，来设置两类Manual Compect的环境变量： |
| <ul> |
| <li>单次Manual Compact： |
| <ul> |
| <li><code class="language-plaintext highlighter-rouge">manual_compact.once.trigger_time</code>：格式为Unix时间戳的秒数，可通过shell命令<code class="language-plaintext highlighter-rouge">date +%s</code>获取当前时间戳。如果LastManualCompactFinishTime旧于该trigger_time，就触发Manual Compaction的执行。</li> |
| <li><code class="language-plaintext highlighter-rouge">manual_compact.once.target_level</code>：用于设置<code class="language-plaintext highlighter-rouge">CompactRangeOptions::target_level</code>。如果不设置，则使用默认值-1。</li> |
| <li><code class="language-plaintext highlighter-rouge">manual_compact.once.bottommost_level_compaction</code>：可设置为<code class="language-plaintext highlighter-rouge">skip</code>或者<code class="language-plaintext highlighter-rouge">force</code>。如果是<code class="language-plaintext highlighter-rouge">skip</code>，则不对最高层做compaction；如果是<code class="language-plaintext highlighter-rouge">force</code>，则强制对最高层做compaction。如果不设置，则默认为<code class="language-plaintext highlighter-rouge">skip</code>。</li> |
| </ul> |
| </li> |
| <li>周期Manual Compact： |
| <ul> |
| <li><code class="language-plaintext highlighter-rouge">manual_compact.periodic.trigger_time</code>：格式为逗号分隔的时钟，譬如<code class="language-plaintext highlighter-rouge">3:00,21:00</code>，表示每一天的3:00和21:00都触发一次Manual Compaction的执行。</li> |
| <li><code class="language-plaintext highlighter-rouge">manual_compact.periodic.target_level</code>：用于设置<code class="language-plaintext highlighter-rouge">CompactRangeOptions::target_level</code>。如果不设置，则使用默认值-1。</li> |
| <li><code class="language-plaintext highlighter-rouge">manual_compact.periodic.bottommost_level_compaction</code>：可设置为<code class="language-plaintext highlighter-rouge">skip</code>或者<code class="language-plaintext highlighter-rouge">force</code>。如果是<code class="language-plaintext highlighter-rouge">skip</code>，则不对最高层做compaction；如果是<code class="language-plaintext highlighter-rouge">force</code>，则强制对最高层做compaction。如果不设置，则默认为<code class="language-plaintext highlighter-rouge">skip</code>。</li> |
| </ul> |
| </li> |
| <li>Manual Compact总开关： |
| <ul> |
| <li><code class="language-plaintext highlighter-rouge">manual_compact.disabled</code>(从v1.9.0版本开始支持)：如果为true，则关闭Manual Compact功能，并且取消正在执行中的Manual Compact动作。如果不设置，默认为false。</li> |
| <li><code class="language-plaintext highlighter-rouge">manual_compact.max_concurrent_running_count</code>(从v1.11.3版本开始支持)：指定最大并发数。实际上，可执行的最大并发数由<code class="language-plaintext highlighter-rouge">该env参数</code>和<code class="language-plaintext highlighter-rouge">服务端MANUAL_COMPACT_THRAD_POOL的线程数</code>共同决定，取两者的较小值。</li> |
| </ul> |
| </li> |
| </ul> |
| </li> |
| </ul> |
| |
| <p>注意：</p> |
| <ul> |
| <li>Manual Compact功能是分派到独立的Compact线程池中执行的，每个线程同一时刻只能处理一个replica的full compaction，因为并发处理量与Compact线程池的线程数量有关，可以通过配置文件的<code class="language-plaintext highlighter-rouge">worker_count</code>进行配置，如果使用Manual Compact比较频繁，建议调大线程数量（譬如设置为cpu core数量接近）： |
| <div class="language-plaintext highlighter-rouge"><div class="highlight"><pre class="highlight"><code>[threadpool.THREAD_POOL_COMPACT] |
| name = compact |
| partitioned = false |
| max_input_queue_length = 128 |
| worker_priority = THREAD_xPRIORITY_NORMAL |
| worker_count = 16 |
| </code></pre></div> </div> |
| </li> |
| <li>Manual Compact属于CPU和IO密集型操作，处理过程中会使CPU使用率长期处于高位，容易对集群的读写性能造成影响，所以<strong>建议在流量低峰时段进行操作</strong>。如果启动后发现读写性能下降影响了业务，可以立即通过设置该表的环境变量<code class="language-plaintext highlighter-rouge">manual_compact.disabled=true</code>来中止。</li> |
| <li>Manual Compact过程中可能需要较多的额外磁盘空间。因为compaction前后文件变化较大，而Pegasus一般又会保留最近3个版本的checkpoint，所以基本上额外需要的磁盘空间量大约等于执行Manual Compact的表的数据存储量。所以，在执行Manual Compact前需<strong>确认集群有足够的存储空间</strong>，同时在执行过程中<strong>关注磁盘空间使用情况</strong>，避免因为磁盘空间耗尽导致集群节点宕机，影响集群可用度。</li> |
| </ul> |
| |
| <h1 id="如何设置">如何设置</h1> |
| |
| <h2 id="通过shell设置">通过shell设置</h2> |
| |
| <p>既然Manual Compact功能是利用<a href="table-env">Table环境变量</a>触发的，那么可以直接通过shell功能的<a href="/zh/overview/shell#set_app_envs">set_app_envs命令</a>来设置。需要设置的环境变量参照上面的描述，环境变量设置后不会立即生效，大约需要等几十秒后才能在所有replica上生效。</p> |
| |
| <p>由于需要设置的环境变量比较多，且对数据格式有要求，所以强烈建议不要自己直接设置，而是通过我们提供的脚本工具来设置，如下所示。</p> |
| |
| <h2 id="通过脚本设置">通过脚本设置</h2> |
| <p>我们提供了一个脚本工具<a href="https://github.com/apache/incubator-pegasus/blob/master/scripts/pegasus_manual_compact.sh">scripts/pegasus_manual_compact.sh</a>来方便地设置，用法：</p> |
| <div class="language-plaintext highlighter-rouge"><div class="highlight"><pre class="highlight"><code>$ ./scripts/pegasus_manual_compact.sh |
| This tool is for manual compact specified table(app). |
| USAGE: ./scripts/pegasus_manual_compact.sh -c cluster -a app-name [-t periodic|once] [-w] [-g trigger_time] [...] |
| Options: |
| -h|--help print help message |
| |
| -c|--cluster <str> cluster meta server list, default is "127.0.0.1:34601,127.0.0.1:34602" |
| |
| -a|--app_name <str> target table(app) name |
| |
| -t|--type <str> manual compact type, should be periodic or once, default is once |
| |
| -w|--wait_only this option is only used when the type is once! |
| not trigger but only wait the last once compact to finish |
| |
| -g|--trigger_time <str> this option is only used when the type is periodic! |
| specify trigger time of periodic compact in 24-hour format, |
| e.g. "3:00,21:00" means 3:00 and 21:00 everyday |
| |
| --target_level <num> number in range of [-1,num_levels], -1 means automatically, default is -1 |
| |
| --bottommost_level_compaction <skip|force> |
| skip or force, default is skip |
| more details: https://github.com/facebook/rocksdb/wiki/Manual-Compaction |
| |
| --max_concurrent_running_count <num> |
| max concurrent running count limit, should be positive integer. |
| if not set, means no limit. |
| |
| for example: |
| |
| 1) Start once type manual compact with default options: |
| |
| ./scripts/pegasus_manual_compact.sh -c 127.0.0.1:34601,127.0.0.1:34602 -a temp |
| |
| 2) Only wait last once type manual compact to finish: |
| |
| ./scripts/pegasus_manual_compact.sh -c 127.0.0.1:34601,127.0.0.1:34602 -a temp -w |
| |
| 3) Config periodic type manual compact with specified options: |
| |
| ./scripts/pegasus_manual_compact.sh -c 127.0.0.1:34601,127.0.0.1:34602 -a temp -t periodic -g 3:00,21:00 \ |
| --target_level 2 --bottommost_level_compaction force |
| </code></pre></div></div> |
| |
| <p>该工具不仅会调用shell命令设置Table环境变量，对于once类型还会等待所有的replica上的操作都执行完成，使用起来十分方便。</p> |
| |
| <p>譬如，在<a href="usage-scenario#支持场景">bulk load</a>完成后执行once manual compact如下：</p> |
| <div class="language-plaintext highlighter-rouge"><div class="highlight"><pre class="highlight"><code>$ ./scripts/pegasus_manual_compact.sh -c 127.0.0.1:34601,127.0.0.1:34602 -a temp |
| </code></pre></div></div> |
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