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itemprop="name">shivnarayan</span></a></div></div></div></div><div class="blogPostData_A2Le">5 min read</div></div></div><ul class="authorTimeTags_oN88 padding--none margin-left--sm tagsWrapperPostPage_VdId"><li class="tag_MgfY tagPostPage_gnvv"><a class="tag_WK-t tagRegular_LXbV" href="/cn/blog/tags/design">design</a></li><li class="tag_MgfY tagPostPage_gnvv"><a class="tag_WK-t tagRegular_LXbV" href="/cn/blog/tags/file-sizing">file sizing</a></li><li class="tag_MgfY tagPostPage_gnvv"><a class="tag_WK-t tagRegular_LXbV" href="/cn/blog/tags/apache-hudi">apache hudi</a></li></ul></header><div class="markdown" itemprop="articleBody"><p>Apache Hudi is a data lake platform technology that provides several functionalities needed to build and manage data lakes. |
| One such key feature that hudi provides is self-managing file sizing so that users don’t need to worry about |
| manual table maintenance. Having a lot of small files will make it harder to achieve good query performance, due to query engines |
| having to open/read/close files way too many times, to plan and execute queries. But for streaming data lake use-cases, |
| inherently ingests are going to end up having smaller volume of writes, which might result in lot of small files if no special handling is done.</p><h2 class="anchor anchorWithStickyNavbar_y2LR" id="during-write-vs-after-write">During Write vs After Write<a class="hash-link" href="#during-write-vs-after-write" title="Direct link to heading"></a></h2><p>Common approaches to writing very small files and then later stitching them together solve for system scalability issues posed |
| by small files but might violate query SLA's by exposing small files to them. In fact, you can easily do so on a Hudi table, |
| by running a clustering operation, as detailed in a <a href="/cn/blog/2021/01/27/hudi-clustering-intro">previous blog</a>. </p><p>In this blog, we discuss file sizing optimizations in Hudi, during the initial write time, so we don't have to effectively |
| re-write all data again, just for file sizing. If you want to have both (a) self managed file sizing and |
| (b) Avoid exposing small files to queries, automatic file sizing feature saves the day.</p><p>Hudi has the ability to maintain a configured target file size, when performing inserts/upsert operations. |
| (Note: bulk_insert operation does not provide this functionality and is designed as a simpler replacement for |
| normal <code>spark.write.parquet</code>).</p><h3 class="anchor anchorWithStickyNavbar_y2LR" id="configs">Configs<a class="hash-link" href="#configs" title="Direct link to heading"></a></h3><p>For illustration purposes, we are going to consider only COPY_ON_WRITE table.</p><p>Configs of interest before we dive into the algorithm:</p><ul><li><a href="/cn/docs/configurations#limitFileSize">Max file size</a>: Max size for a given data file. Hudi will try to maintain file sizes to this configured value <br></li><li><a href="/cn/docs/configurations#compactionSmallFileSize">Soft file limit</a>: Max file size below which a given data file is considered to a small file <br></li><li><a href="/cn/docs/configurations#insertSplitSize">Insert split size</a>: Number of inserts grouped for a single partition. This value should match |
| the number of records in a single file (you can determine based on max file size and per record size)</li></ul><p>For instance, if your first config value is 120MB and 2nd config value is set to 100MB, any file whose size is < 100MB |
| would be considered a small file.</p><p>If you wish to turn off this feature, set the config value for soft file limit to 0.</p><h3 class="anchor anchorWithStickyNavbar_y2LR" id="example">Example<a class="hash-link" href="#example" title="Direct link to heading"></a></h3><p>Let’s say this is the layout of data files for a given partition.</p><p><img alt="Initial layout" src="/cn/assets/images/initial_layout-ba5e4c454e6d2328f74dfc5e9fa2966a.png"> |
| <em>Figure: Initial data file sizes for a given partition of interest</em></p><p>Let’s assume the configured values for max file size and small file size limit are 120MB and 100MB. File_1’s current |
| size is 40MB, File_2’s size is 80MB, File_3’s size is 90MB, File_4’s size is 130MB and File_5’s size is 105MB. Let’s see |
| what happens when a new write happens. </p><p><strong>Step 1:</strong> Assigning updates to files. In this step, We look up the index to find the tagged location and records are |
| assigned to respective files. Note that we assume updates are only going to increase the file size and that would simply result |
| in a much bigger file. When updates lower the file size (by say, nulling out lot of fields), then a subsequent write will deem |
| it a small file.</p><p><strong>Step 2:</strong> Determine small files for each partition path. The soft file limit config value will be leveraged here |
| to determine eligible small files. In our example, given the config value is set to 100MB, the small files are File_1(40MB) |
| and File_2(80MB) and file_3’s (90MB).</p><p><strong>Step 3:</strong> Once small files are determined, incoming inserts are assigned to them so that they reach their max capacity of |
| 120MB. File_1 will be ingested with 80MB worth of inserts, file_2 will be ingested with 40MB worth of inserts and |
| File_3 will be ingested with 30MB worth of inserts.</p><p><img alt="Bin packing small files" src="/cn/assets/images/bin_packing_existing_data_files-021f5b531f048bfd9cc1230f93c22a71.png"> |
| <em>Figure: Incoming records are bin packed to existing small files</em></p><p><strong>Step 4:</strong> Once all small files are bin packed to its max capacity and if there are pending inserts unassigned, new file |
| groups/data files are created and inserts are assigned to them. Number of records per new data file is determined from insert split |
| size config. Assuming the insert split size is configured to 120k records, if there are 300k remaining records, 3 new |
| files will be created in which 2 of them (File_6 and File_7) will be filled with 120k records and the last one (File_8) |
| will be filled with 60k records (assuming each record is 1000 bytes). In future ingestions, 3rd new file will be |
| considered as a small file to be packed with more data.</p><p><img alt="Assigning to new files" src="/cn/assets/images/adding_new_files-13e5a1cf0c213c07a412b09a29be4e3d.png"> |
| <em>Figure: Remaining records are assigned to new files</em></p><p>Hudi leverages mechanisms such as custom partitioning for optimized record distribution to different files, executing |
| the algorithm above. After this round of ingestion is complete, all files except File_8 are nicely sized to the optimum size. |
| This process is followed during every ingestion to ensure there are no small files in your Hudi tables. </p><p>Hopefully the blog gave you an overview into how hudi manages small files and assists in boosting your query performance.</p></div><footer0 class="row docusaurus-mt-lg blogPostDetailsFull_2lop"><div class="col margin-top--sm"><a href="https://github.com/apache/hudi/edit/asf-site/website/blog/blog/2021-03-01-hudi-file-sizing.md" target="_blank" rel="noreferrer noopener" class="theme-edit-this-page"><svg fill="currentColor" height="20" width="20" viewBox="0 0 40 40" class="iconEdit_mS5F" aria-hidden="true"><g><path d="m34.5 11.7l-3 3.1-6.3-6.3 3.1-3q0.5-0.5 1.2-0.5t1.1 0.5l3.9 3.9q0.5 0.4 0.5 1.1t-0.5 1.2z m-29.5 17.1l18.4-18.5 6.3 6.3-18.4 18.4h-6.3v-6.2z"></path></g></svg>Edit this page</a></div></footer0></article><nav class="pagination-nav docusaurus-mt-lg" aria-label="Blog post page navigation"><div class="pagination-nav__item"><a class="pagination-nav__link" 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