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|         <input class="md-nav__toggle md-toggle " type="checkbox" id="__nav_6" checked> | |
|          | |
|            | |
|           <label class="md-nav__link" for="__nav_6" id="__nav_6_label" tabindex="0"> | |
|              | |
|    | |
|    | |
|   <span class="md-ellipsis"> | |
|     Config | |
|      | |
|   </span> | |
|    | |
| 
 | |
|             <span class="md-nav__icon md-icon"></span> | |
|           </label> | |
|          | |
|         <nav class="md-nav" data-md-level="1" aria-labelledby="__nav_6_label" aria-expanded="true"> | |
|           <label class="md-nav__title" for="__nav_6"> | |
|             <span class="md-nav__icon md-icon"></span> | |
|             Config | |
|           </label> | |
|           <ul class="md-nav__list" data-md-scrollfix> | |
|              | |
|                | |
|                  | |
|    | |
|    | |
|    | |
|    | |
|     <li class="md-nav__item"> | |
|       <a href="../options/" class="md-nav__link"> | |
|          | |
|    | |
|    | |
|   <span class="md-ellipsis"> | |
|     Options | |
|      | |
|   </span> | |
|    | |
| 
 | |
|       </a> | |
|     </li> | |
|    | |
| 
 | |
|                | |
|              | |
|                | |
|                  | |
|    | |
|    | |
|    | |
|    | |
|     <li class="md-nav__item"> | |
|       <a href="../deprecated_options/" class="md-nav__link"> | |
|          | |
|    | |
|    | |
|   <span class="md-ellipsis"> | |
|     Deprecated Options | |
|      | |
|   </span> | |
|    | |
| 
 | |
|       </a> | |
|     </li> | |
|    | |
| 
 | |
|                | |
|              | |
|                | |
|                  | |
|    | |
|    | |
|    | |
|    | |
|     <li class="md-nav__item"> | |
|       <a href="../branches/" class="md-nav__link"> | |
|          | |
|    | |
|    | |
|   <span class="md-ellipsis"> | |
|     branches | |
|      | |
|   </span> | |
|    | |
| 
 | |
|       </a> | |
|     </li> | |
|    | |
| 
 | |
|                | |
|              | |
|                | |
|                  | |
|    | |
|    | |
|    | |
|    | |
|     <li class="md-nav__item"> | |
|       <a href="../branches-mount-timeout/" class="md-nav__link"> | |
|          | |
|    | |
|    | |
|   <span class="md-ellipsis"> | |
|     branches-mount-timeout | |
|      | |
|   </span> | |
|    | |
| 
 | |
|       </a> | |
|     </li> | |
|    | |
| 
 | |
|                | |
|              | |
|                | |
|                  | |
|    | |
|    | |
|    | |
|    | |
|     <li class="md-nav__item"> | |
|       <a href="../functions_categories_policies/" class="md-nav__link"> | |
|          | |
|    | |
|    | |
|   <span class="md-ellipsis"> | |
|     functions, categories and policies | |
|      | |
|   </span> | |
|    | |
| 
 | |
|       </a> | |
|     </li> | |
|    | |
| 
 | |
|                | |
|              | |
|                | |
|                  | |
|    | |
|    | |
|    | |
|    | |
|     <li class="md-nav__item"> | |
|       <a href="../minfreespace/" class="md-nav__link"> | |
|          | |
|    | |
|    | |
|   <span class="md-ellipsis"> | |
|     minfreespace | |
|      | |
|   </span> | |
|    | |
| 
 | |
|       </a> | |
|     </li> | |
|    | |
| 
 | |
|                | |
|              | |
|                | |
|                  | |
|    | |
|    | |
|    | |
|    | |
|     <li class="md-nav__item"> | |
|       <a href="../func_readdir/" class="md-nav__link"> | |
|          | |
|    | |
|    | |
|   <span class="md-ellipsis"> | |
|     func.readdir | |
|      | |
|   </span> | |
|    | |
| 
 | |
|       </a> | |
|     </li> | |
|    | |
| 
 | |
|                | |
|              | |
|                | |
|                  | |
|    | |
|    | |
|    | |
|    | |
|     <li class="md-nav__item"> | |
|       <a href="../rename_and_link/" class="md-nav__link"> | |
|          | |
|    | |
|    | |
|   <span class="md-ellipsis"> | |
|     rename and link | |
|      | |
|   </span> | |
|    | |
| 
 | |
|       </a> | |
|     </li> | |
|    | |
| 
 | |
|                | |
|              | |
|                | |
|                  | |
|    | |
|    | |
|    | |
|    | |
|     <li class="md-nav__item"> | |
|       <a href="../moveonenospc/" class="md-nav__link"> | |
|          | |
|    | |
|    | |
|   <span class="md-ellipsis"> | |
|     moveonenospc | |
|      | |
|   </span> | |
|    | |
| 
 | |
|       </a> | |
|     </li> | |
|    | |
| 
 | |
|                | |
|              | |
|                | |
|                  | |
|    | |
|    | |
|      | |
|    | |
|    | |
|    | |
|     <li class="md-nav__item md-nav__item--active"> | |
|        | |
|       <input class="md-nav__toggle md-toggle" type="checkbox" id="__toc"> | |
|        | |
|        | |
|          | |
|        | |
|        | |
|         <label class="md-nav__link md-nav__link--active" for="__toc"> | |
|            | |
|    | |
|    | |
|   <span class="md-ellipsis"> | |
|     caching | |
|      | |
|   </span> | |
|    | |
| 
 | |
|           <span class="md-nav__icon md-icon"></span> | |
|         </label> | |
|        | |
|       <a href="./" class="md-nav__link md-nav__link--active"> | |
|          | |
|    | |
|    | |
|   <span class="md-ellipsis"> | |
|     caching | |
|      | |
|   </span> | |
|    | |
| 
 | |
|       </a> | |
|        | |
|          | |
| 
 | |
| <nav class="md-nav md-nav--secondary" aria-label="Table of contents"> | |
|    | |
|    | |
|    | |
|      | |
|    | |
|    | |
|     <label class="md-nav__title" for="__toc"> | |
|       <span class="md-nav__icon md-icon"></span> | |
|       Table of contents | |
|     </label> | |
|     <ul class="md-nav__list" data-md-component="toc" data-md-scrollfix> | |
|        | |
|         <li class="md-nav__item"> | |
|   <a href="#cachefiles" class="md-nav__link"> | |
|     <span class="md-ellipsis"> | |
|       cache.files | |
|     </span> | |
|   </a> | |
|    | |
| </li> | |
|        | |
|         <li class="md-nav__item"> | |
|   <a href="#cacheentry" class="md-nav__link"> | |
|     <span class="md-ellipsis"> | |
|       cache.entry | |
|     </span> | |
|   </a> | |
|    | |
| </li> | |
|        | |
|         <li class="md-nav__item"> | |
|   <a href="#cachenegative_entry" class="md-nav__link"> | |
|     <span class="md-ellipsis"> | |
|       cache.negative_entry | |
|     </span> | |
|   </a> | |
|    | |
| </li> | |
|        | |
|         <li class="md-nav__item"> | |
|   <a href="#cacheattr" class="md-nav__link"> | |
|     <span class="md-ellipsis"> | |
|       cache.attr | |
|     </span> | |
|   </a> | |
|    | |
| </li> | |
|        | |
|         <li class="md-nav__item"> | |
|   <a href="#cachestatfs" class="md-nav__link"> | |
|     <span class="md-ellipsis"> | |
|       cache.statfs | |
|     </span> | |
|   </a> | |
|    | |
| </li> | |
|        | |
|         <li class="md-nav__item"> | |
|   <a href="#cachesymlinks" class="md-nav__link"> | |
|     <span class="md-ellipsis"> | |
|       cache.symlinks | |
|     </span> | |
|   </a> | |
|    | |
| </li> | |
|        | |
|         <li class="md-nav__item"> | |
|   <a href="#cachereaddir" class="md-nav__link"> | |
|     <span class="md-ellipsis"> | |
|       cache.readdir | |
|     </span> | |
|   </a> | |
|    | |
| </li> | |
|        | |
|         <li class="md-nav__item"> | |
|   <a href="#cachewriteback" class="md-nav__link"> | |
|     <span class="md-ellipsis"> | |
|       cache.writeback | |
|     </span> | |
|   </a> | |
|    | |
| </li> | |
|        | |
|     </ul> | |
|    | |
| </nav> | |
|        | |
|     </li> | |
|    | |
| 
 | |
|                | |
|              | |
|                | |
|                  | |
|    | |
|    | |
|    | |
|    | |
|     <li class="md-nav__item"> | |
|       <a href="../passthrough/" class="md-nav__link"> | |
|          | |
|    | |
|    | |
|   <span class="md-ellipsis"> | |
|     passthrough | |
|      | |
|   </span> | |
|    | |
| 
 | |
|       </a> | |
|     </li> | |
|    | |
| 
 | |
|                | |
|              | |
|                | |
|                  | |
|    | |
|    | |
|    | |
|    | |
|     <li class="md-nav__item"> | |
|       <a href="../readahead/" class="md-nav__link"> | |
|          | |
|    | |
|    | |
|   <span class="md-ellipsis"> | |
|     readahead | |
|      | |
|   </span> | |
|    | |
| 
 | |
|       </a> | |
|     </li> | |
|    | |
| 
 | |
|                | |
|              | |
|                | |
|                  | |
|    | |
|    | |
|    | |
|    | |
|     <li class="md-nav__item"> | |
|       <a href="../inodecalc/" class="md-nav__link"> | |
|          | |
|    | |
|    | |
|   <span class="md-ellipsis"> | |
|     inodecalc | |
|      | |
|   </span> | |
|    | |
| 
 | |
|       </a> | |
|     </li> | |
|    | |
| 
 | |
|                | |
|              | |
|                | |
|                  | |
|    | |
|    | |
|    | |
|    | |
|     <li class="md-nav__item"> | |
|       <a href="../threads/" class="md-nav__link"> | |
|          | |
|    | |
|    | |
|   <span class="md-ellipsis"> | |
|     threads | |
|      | |
|   </span> | |
|    | |
| 
 | |
|       </a> | |
|     </li> | |
|    | |
| 
 | |
|                | |
|              | |
|                | |
|                  | |
|    | |
|    | |
|    | |
|    | |
|     <li class="md-nav__item"> | |
|       <a href="../pin-threads/" class="md-nav__link"> | |
|          | |
|    | |
|    | |
|   <span class="md-ellipsis"> | |
|     pin-threads | |
|      | |
|   </span> | |
|    | |
| 
 | |
|       </a> | |
|     </li> | |
|    | |
| 
 | |
|                | |
|              | |
|                | |
|                  | |
|    | |
|    | |
|    | |
|    | |
|     <li class="md-nav__item"> | |
|       <a href="../link_cow/" class="md-nav__link"> | |
|          | |
|    | |
|    | |
|   <span class="md-ellipsis"> | |
|     link_cow | |
|      | |
|   </span> | |
|    | |
| 
 | |
|       </a> | |
|     </li> | |
|    | |
| 
 | |
|                | |
|              | |
|                | |
|                  | |
|    | |
|    | |
|    | |
|    | |
|     <li class="md-nav__item"> | |
|       <a href="../fuse_msg_size/" class="md-nav__link"> | |
|          | |
|    | |
|    | |
|   <span class="md-ellipsis"> | |
|     fuse_msg_size | |
|      | |
|   </span> | |
|    | |
| 
 | |
|       </a> | |
|     </li> | |
|    | |
| 
 | |
|                | |
|              | |
|                | |
|                  | |
|    | |
|    | |
|    | |
|    | |
|     <li class="md-nav__item"> | |
|       <a href="../follow-symlinks/" class="md-nav__link"> | |
|          | |
|    | |
|    | |
|   <span class="md-ellipsis"> | |
|     follow-symlinks | |
|      | |
|   </span> | |
|    | |
| 
 | |
|       </a> | |
|     </li> | |
|    | |
| 
 | |
|                | |
|              | |
|                | |
|                  | |
|    | |
|    | |
|    | |
|    | |
|     <li class="md-nav__item"> | |
|       <a href="../link-exdev/" class="md-nav__link"> | |
|          | |
|    | |
|    | |
|   <span class="md-ellipsis"> | |
|     link-exdev | |
|      | |
|   </span> | |
|    | |
| 
 | |
|       </a> | |
|     </li> | |
|    | |
| 
 | |
|                | |
|              | |
|                | |
|                  | |
|    | |
|    | |
|    | |
|    | |
|     <li class="md-nav__item"> | |
|       <a href="../rename-exdev/" class="md-nav__link"> | |
|          | |
|    | |
|    | |
|   <span class="md-ellipsis"> | |
|     rename-exdev | |
|      | |
|   </span> | |
|    | |
| 
 | |
|       </a> | |
|     </li> | |
|    | |
| 
 | |
|                | |
|              | |
|                | |
|                  | |
|    | |
|    | |
|    | |
|    | |
|     <li class="md-nav__item"> | |
|       <a href="../symlinkify/" class="md-nav__link"> | |
|          | |
|    | |
|    | |
|   <span class="md-ellipsis"> | |
|     symlinkify | |
|      | |
|   </span> | |
|    | |
| 
 | |
|       </a> | |
|     </li> | |
|    | |
| 
 | |
|                | |
|              | |
|                | |
|                  | |
|    | |
|    | |
|    | |
|    | |
|     <li class="md-nav__item"> | |
|       <a href="../nullrw/" class="md-nav__link"> | |
|          | |
|    | |
|    | |
|   <span class="md-ellipsis"> | |
|     nullrw | |
|      | |
|   </span> | |
|    | |
| 
 | |
|       </a> | |
|     </li> | |
|    | |
| 
 | |
|                | |
|              | |
|                | |
|                  | |
|    | |
|    | |
|    | |
|    | |
|     <li class="md-nav__item"> | |
|       <a href="../xattr/" class="md-nav__link"> | |
|          | |
|    | |
|    | |
|   <span class="md-ellipsis"> | |
|     xattr | |
|      | |
|   </span> | |
|    | |
| 
 | |
|       </a> | |
|     </li> | |
|    | |
| 
 | |
|                | |
|              | |
|                | |
|                  | |
|    | |
|    | |
|    | |
|    | |
|     <li class="md-nav__item"> | |
|       <a href="../nfsopenhack/" class="md-nav__link"> | |
|          | |
|    | |
|    | |
|   <span class="md-ellipsis"> | |
|     nfsopenhack | |
|      | |
|   </span> | |
|    | |
| 
 | |
|       </a> | |
|     </li> | |
|    | |
| 
 | |
|                | |
|              | |
|                | |
|                  | |
|    | |
|    | |
|    | |
|    | |
|     <li class="md-nav__item"> | |
|       <a href="../statfs/" class="md-nav__link"> | |
|          | |
|    | |
|    | |
|   <span class="md-ellipsis"> | |
|     statfs / statvfs | |
|      | |
|   </span> | |
|    | |
| 
 | |
|       </a> | |
|     </li> | |
|    | |
| 
 | |
|                | |
|              | |
|                | |
|                  | |
|    | |
|    | |
|    | |
|    | |
|     <li class="md-nav__item"> | |
|       <a href="../flush-on-close/" class="md-nav__link"> | |
|          | |
|    | |
|    | |
|   <span class="md-ellipsis"> | |
|     flush-on-close | |
|      | |
|   </span> | |
|    | |
| 
 | |
|       </a> | |
|     </li> | |
|    | |
| 
 | |
|                | |
|              | |
|                | |
|                  | |
|    | |
|    | |
|    | |
|    | |
|     <li class="md-nav__item"> | |
|       <a href="../export-support/" class="md-nav__link"> | |
|          | |
|    | |
|    | |
|   <span class="md-ellipsis"> | |
|     export-support | |
|      | |
|   </span> | |
|    | |
| 
 | |
|       </a> | |
|     </li> | |
|    | |
| 
 | |
|                | |
|              | |
|                | |
|                  | |
|    | |
|    | |
|    | |
|    | |
|     <li class="md-nav__item"> | |
|       <a href="../kernel-permissions-check/" class="md-nav__link"> | |
|          | |
|    | |
|    | |
|   <span class="md-ellipsis"> | |
|     kernel-permissions-check | |
|      | |
|   </span> | |
|    | |
| 
 | |
|       </a> | |
|     </li> | |
|    | |
| 
 | |
|                | |
|              | |
|           </ul> | |
|         </nav> | |
|        | |
|     </li> | |
|    | |
| 
 | |
|      | |
|        | |
|        | |
|    | |
|    | |
|    | |
|    | |
|     <li class="md-nav__item"> | |
|       <a href="../../error_handling_and_logging/" class="md-nav__link"> | |
|          | |
|    | |
|    | |
|   <span class="md-ellipsis"> | |
|     Error Handling and Logging | |
|      | |
|   </span> | |
|    | |
| 
 | |
|       </a> | |
|     </li> | |
|    | |
| 
 | |
|      | |
|        | |
|        | |
|    | |
|    | |
|    | |
|    | |
|     <li class="md-nav__item"> | |
|       <a href="../../resource_usage/" class="md-nav__link"> | |
|          | |
|    | |
|    | |
|   <span class="md-ellipsis"> | |
|     Resource Usage and Management | |
|      | |
|   </span> | |
|    | |
| 
 | |
|       </a> | |
|     </li> | |
|    | |
| 
 | |
|      | |
|        | |
|        | |
|    | |
|    | |
|    | |
|    | |
|     <li class="md-nav__item"> | |
|       <a href="../../runtime_interface/" class="md-nav__link"> | |
|          | |
|    | |
|    | |
|   <span class="md-ellipsis"> | |
|     Runtime Interface | |
|      | |
|   </span> | |
|    | |
| 
 | |
|       </a> | |
|     </li> | |
|    | |
| 
 | |
|      | |
|        | |
|        | |
|    | |
|    | |
|    | |
|    | |
|     <li class="md-nav__item"> | |
|       <a href="../../remote_filesystems/" class="md-nav__link"> | |
|          | |
|    | |
|    | |
|   <span class="md-ellipsis"> | |
|     Remote Filesystems | |
|      | |
|   </span> | |
|    | |
| 
 | |
|       </a> | |
|     </li> | |
|    | |
| 
 | |
|      | |
|        | |
|        | |
|    | |
|    | |
|    | |
|    | |
|     <li class="md-nav__item"> | |
|       <a href="../../tips_notes/" class="md-nav__link"> | |
|          | |
|    | |
|    | |
|   <span class="md-ellipsis"> | |
|     Tips and Notes | |
|      | |
|   </span> | |
|    | |
| 
 | |
|       </a> | |
|     </li> | |
|    | |
| 
 | |
|      | |
|        | |
|        | |
|    | |
|    | |
|    | |
|    | |
|     <li class="md-nav__item"> | |
|       <a href="../../known_issues_bugs/" class="md-nav__link"> | |
|          | |
|    | |
|    | |
|   <span class="md-ellipsis"> | |
|     Known Issues and Bugs | |
|      | |
|   </span> | |
|    | |
| 
 | |
|       </a> | |
|     </li> | |
|    | |
| 
 | |
|      | |
|        | |
|        | |
|    | |
|    | |
|    | |
|    | |
|     <li class="md-nav__item"> | |
|       <a href="../../project_comparisons/" class="md-nav__link"> | |
|          | |
|    | |
|    | |
|   <span class="md-ellipsis"> | |
|     Project Comparisons | |
|      | |
|   </span> | |
|    | |
| 
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| <h1 id="caching">caching</h1> | |
| <h2 id="cachefiles">cache.files</h2> | |
| <p>Controls how <a href="https://en.wikipedia.org/wiki/Page_cache">page caching</a> | |
| works for mergerfs itself. Not the underlying filesystems.</p> | |
| <ul> | |
| <li><code>cache.files=off</code>: Disables page caching for mergerfs.</li> | |
| <li><code>cache.files=partial</code>: Enables page caching. Files are cached | |
|   while open.</li> | |
| <li><code>cache.files=full</code>: Enables page caching. Files are cached across | |
|   opens.</li> | |
| <li><code>cache.files=auto-full</code>: Enables page caching. Files are cached | |
|   across opens if mtime and size are unchanged since previous | |
|   open. Cache is dropped if mtime or size change on open.</li> | |
| <li><code>cache.files=per-process</code>: Enable page caching (equivalent to | |
|   <code>cache.files=partial</code>) only for processes whose 'comm' name matches | |
|   one of the values defined in cache.files.process-names. If the name | |
|   does not match the file open is equivalent to <code>cache.files=off</code>.</li> | |
| </ul> | |
| <p>Generally, enabling the page cache actually <em>harms</em> | |
| performance<sup id="fnref:1"><a class="footnote-ref" href="#fn:1">1</a></sup>. In part because it can lead to buffer bloat due to | |
| the kernel caching both the underlying filesystem's file content as | |
| well as the file through mergerfs. However, if you want to confirm | |
| performance differences it is recommended that you perform some | |
| benchmark to confirm which option works best for your setup.</p> | |
| <p>Why then would you want to enable page caching if it consumes ~2x the | |
| RAM as normal and is on average slower? Because it is the only way to | |
| support | |
| <a href="https://man7.org/linux/man-pages/man2/mmap.2.html">mmap</a>. <code>mmap</code> is a | |
| way for programs to treat a file as if it is a contiguous RAM buffer | |
| which is regularly used by a number of programs such as those that | |
| leverage <strong>sqlite3</strong>. Despite <code>mmap</code> not being supported by all | |
| filesystems it is unfortunately common for software to not have an | |
| option to use regular file IO instead of <code>mmap</code>.</p> | |
| <p>The good thing is that in Linux v6.6<sup id="fnref:2"><a class="footnote-ref" href="#fn:2">2</a></sup> and above FUSE can now | |
| transparently enable page caching when mmap is requested. This means | |
| it should be safe to set <code>cache.files=off</code>. However, on Linux v6.5 and | |
| below you will need to configure <code>cache.files</code> as you need.</p> | |
| <p>If <a href="../passthrough/">passthrough</a> is enabled so must be page | |
| caching. mergerfs will set <code>cache.files=auto-full</code> if <code>passthrough</code> is | |
| enabled. And when using <code>passthrough</code> the there is no double page | |
| caching since it is in fact passing through the IO.</p> | |
| <h2 id="cacheentry">cache.entry</h2> | |
| <ul> | |
| <li><code>cache.entry=UINT</code>: Sets the number of seconds to cache | |
|   entry queries. Defaults to <code>1</code>.</li> | |
| </ul> | |
| <p>The kernel must ask mergerfs about the existence of files. The entry | |
| cache caches that those details which limits the number of requests | |
| sent to mergerfs.</p> | |
| <p>The risk of setting this value, as with most any cache, is related to | |
| <a href="https://en.wikipedia.org/wiki/Out-of-band">out-of-band</a> changes. If | |
| the filesystems are changed outside mergerfs there is a risk of files | |
| which have been removed continuing to show as available. It will fail | |
| gracefully if a phantom file is actioned on in some way so there is | |
| little risk in setting the value much higher. Especially if there are | |
| no out-of-band changes.</p> | |
| <h2 id="cachenegative_entry">cache.negative_entry</h2> | |
| <ul> | |
| <li><code>cache.negative_entry=UINT</code>: Sets the number of seconds to cache | |
|   negative entry queries. Defaults to <code>1</code>.</li> | |
| </ul> | |
| <p>This is a cache for negative entry query responses. Such as when a | |
| file which does not exist is referenced.</p> | |
| <p>The risk of setting this value, as with most any cache, is related to | |
| <a href="https://en.wikipedia.org/wiki/Out-of-band">out-of-band</a> changes. If | |
| the filesystems are changed outside mergerfs there is a risk of files | |
| which have been added outside mergerfs not appearing correctly till | |
| the cache entry times out if there had been a request for the same | |
| name within mergerfs which didn't exist. This is mostly an | |
| inconvenience.</p> | |
| <h2 id="cacheattr">cache.attr</h2> | |
| <ul> | |
| <li><code>cache.attr=UINT</code>: Sets the number of seconds to cache file | |
|   attributes. Defaults to <code>1</code>.</li> | |
| </ul> | |
| <p>This is a cache for file attributes and metadata such as that which is | |
| collected by the | |
| <a href="https://man7.org/linux/man-pages/man2/stat.2.html">stat</a> system call | |
| which is used when you run commands such as <code>find</code> or <code>ls -lh</code>. </p> | |
| <p>As with other caches the risk of enabling the attribute cache is if | |
| changes are made to the file out-of-band there could be | |
| inconsistencies between the actual file and the cached details which | |
| could result in different issues depending on how the data is used. If | |
| the simultaneous writing of a file from inside and outside is unlikely | |
| then you should be safe. That said any simultaneous, uncoordinated | |
| manipulation of a file can lead to unexpected results.</p> | |
| <h2 id="cachestatfs">cache.statfs</h2> | |
| <ul> | |
| <li><code>cache.statfs=UINT</code>: Sets the number of seconds to cache <code>statfs</code> | |
|   calls used by policies. Defaults to <code>0</code>.</li> | |
| </ul> | |
| <p>A number of policies require looking up the available space of the | |
| branches being considered. This is accomplished by calling | |
| <a href="https://man7.org/linux/man-pages/man2/statfs.2.html">statfs</a>. This | |
| call however is a bit expensive so this cache reduces the overhead by | |
| limiting how often the calls are actually made.</p> | |
| <p>This will mean that if the available space of branches changed | |
| somewhat rapidly there is a risk of <code>create</code> or <code>mkdir</code> calls made | |
| within the timeout period ending up on the same branch. This however | |
| should even itself out over time.</p> | |
| <h2 id="cachesymlinks">cache.symlinks</h2> | |
| <ul> | |
| <li><code>cache.symlinks=BOOL</code>: Enable kernel caching of symlink | |
|   values. Defaults to <code>false</code>.</li> | |
| </ul> | |
| <p>As of Linux v4.20 there is an ability to cache the value of symlinks | |
| so that the kernel does not need to make a request to mergerfs every | |
| single time a | |
| <a href="https://man7.org/linux/man-pages/man2/readlink.2.html">readlink</a> | |
| request is made. While not a common usage pattern, if software very | |
| regularly queries symlink values, the use of this cache could | |
| significantly improve performance.</p> | |
| <p>mergerfs will not error if the kernel used does not support symlink | |
| caching.</p> | |
| <p>As with other caches the main risk in enabling it is if you are | |
| manipulating symlinks from both within and without the mergerfs | |
| mount. Should the value be changed outside of mergerfs then it will | |
| not be reflected in the mergerfs mount till the cached value is | |
| invalidated.</p> | |
| <h2 id="cachereaddir">cache.readdir</h2> | |
| <ul> | |
| <li><code>cache.readdir=BOOL</code>: Enable kernel caching of readdir | |
|   results. Defaults to <code>false</code>.</li> | |
| </ul> | |
| <p>As of Linux v4.20 it supports readdir caching. This can have a | |
| significant impact on directory traversal. Especially when combined | |
| with entry (<code>cache.entry</code>) and attribute (<code>cache.attr</code>) caching. If | |
| the kernel doesn't support readdir caching setting the option to true | |
| has no effect. This option is configurable at runtime via xattr | |
| user.mergerfs.cache.readdir.</p> | |
| <h2 id="cachewriteback">cache.writeback</h2> | |
| <ul> | |
| <li><code>cache.writeback=BOOL</code>: Enable writeback cache. Defaults to <code>false</code>.</li> | |
| </ul> | |
| <p>When <code>cache.files</code> is enabled the default is for it to perform | |
| writethrough caching. This behavior won't help improve performance as | |
| each write still goes one for one through the filesystem. By enabling | |
| the FUSE writeback cache small writes <em>may</em> be aggregated by the | |
| kernel and then sent to mergerfs as one larger request. This can | |
| greatly improve the throughput for apps which write to files | |
| inefficiently. The amount the kernel can aggregate is limited by the | |
| size of a FUSE message. Read the fuse_msg_size section for more | |
| details.</p> | |
| <p>There is a side effect as a result of enabling writeback | |
| caching. Underlying files won't ever be opened with O_APPEND or | |
| O_WRONLY. The former because the kernel then manages append mode and | |
| the latter because the kernel may request file data from mergerfs to | |
| populate the write cache. The O_APPEND change means that if a file is | |
| changed outside of mergerfs it could lead to corruption as the kernel | |
| won't know the end of the file has changed. That said any time you use | |
| caching you should keep from writing to the same file outside of | |
| mergerfs at the same time.</p> | |
| <p>Note that if an application is properly sizing writes then writeback | |
| caching will have little or no effect. It will only help with writes | |
| of sizes below the FUSE message size (128K on older kernels, 1M on | |
| newer). Even then its effectiveness might not be great. Given the side | |
| effects of enabling this feature it is recommended that its benefits | |
| be proved out with benchmarks.</p> | |
| <div class="footnote"> | |
| <hr /> | |
| <ol> | |
| <li id="fn:1"> | |
| <p>This is not unique to mergerfs and affects all FUSE | |
| filesystems. It is something that the FUSE community hopes to | |
| investigate at some point but as of early 2025 there are a number | |
| of major reworking going on with FUSE which needs to be finished | |
| first. <a class="footnote-backref" href="#fnref:1" title="Jump back to footnote 1 in the text">↩</a></p> | |
| </li> | |
| <li id="fn:2"> | |
| <p><a href="https://kernelnewbies.org/Linux_6.6#FUSE">https://kernelnewbies.org/Linux_6.6#FUSE</a> <a class="footnote-backref" href="#fnref:2" title="Jump back to footnote 2 in the text">↩</a></p> | |
| </li> | |
| </ol> | |
| </div> | |
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