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| <section id="union-file-system"> |
| <h1>Union File System<a class="headerlink" href="#union-file-system" title="Permalink to this heading"></a></h1> |
| <section id="overview"> |
| <h2>Overview<a class="headerlink" href="#overview" title="Permalink to this heading"></a></h2> |
| <p>This directory contains the NuttX Union File System. The Union file |
| system is provides a mechanism to overlay two different, mounted file |
| systems so that they appear as one. In general this works like this:</p> |
| <blockquote> |
| <div><ol class="arabic simple"> |
| <li><p>Mount file system 1 at some location, say <code class="docutils literal notranslate"><span class="pre">/mnt/file1</span></code></p></li> |
| <li><p>Mount file system 2 at some location, say <code class="docutils literal notranslate"><span class="pre">/mnt/file2</span></code></p></li> |
| <li><p>Call <code class="docutils literal notranslate"><span class="pre">mount()</span></code> to combine and overly <code class="docutils literal notranslate"><span class="pre">/mnt/file1</span></code> and <code class="docutils literal notranslate"><span class="pre">mnt/file2</span></code> |
| as a new mount point, say <code class="docutils literal notranslate"><span class="pre">/mnt/unionfs</span></code>.</p></li> |
| </ol> |
| </div></blockquote> |
| <p><code class="docutils literal notranslate"><span class="pre">/mnt/file1</span></code> and <code class="docutils literal notranslate"><span class="pre">/mnt/file2</span></code> will disappear and be replaced by the single |
| mountpoint <code class="docutils literal notranslate"><span class="pre">/mnt/unionfs</span></code>. The previous contents under <code class="docutils literal notranslate"><span class="pre">/mnt/file1</span></code> and |
| <code class="docutils literal notranslate"><span class="pre">/mnt/file2</span></code> will appear merged under <code class="docutils literal notranslate"><span class="pre">/mnt/unionfs</span></code>. Files at the same |
| relative path in file system1 will take presence. If another file of the |
| same name and same relative location exists in file system 2, it will |
| not be visible because it will be occluded by the file in file system1.</p> |
| <p>See <code class="docutils literal notranslate"><span class="pre">include/nutts/unionfs.h</span></code> for additional information.</p> |
| <p>The Union File System is enabled by selecting the <code class="docutils literal notranslate"><span class="pre">CONFIG_FS_UNIONFS</span></code> option |
| in the NuttX configuration file.</p> |
| <p>Disclaimer: This Union File System was certainly inspired by UnionFS |
| (<a class="reference external" href="http://en.wikipedia.org/wiki/UnionFS">http://en.wikipedia.org/wiki/UnionFS</a>) and the similarity in naming is |
| unavoidable. However, other than that, the NuttX Union File System |
| has no relationship with the UnioinFS project in specification, usage, |
| design, or implementation.</p> |
| </section> |
| <section id="uses-of-the-union-file-system"> |
| <h2>Uses of the Union File System<a class="headerlink" href="#uses-of-the-union-file-system" title="Permalink to this heading"></a></h2> |
| <p>The original motivation for this file was for the use of the built-in |
| function file system (BINFS) with a web server. In that case, the built |
| in functions provide CGI programs. But the BINFS file system cannot hold |
| content. Fixed content would need to be retained in a more standard file |
| system such as ROMFS. With this Union File System, you can overly the |
| BINFS mountpoint on the ROMFS mountpoint, providing a single directory |
| that appears to contain the executables from the BINFS file system along |
| with the web content from the ROMFS file system.</p> |
| <p>Another possible use for the Union File System could be to augment or |
| replace files in a FLASH file system. For example, suppose that you have |
| a product that ships with content in a ROMFS file system provided by the |
| on-board FLASH. Later, you overlay that ROMFS file system with additional |
| files from an SD card by using the Union File System to overlay, and |
| perhaps replace, the ROMFS files.</p> |
| <p>Another use case might be to overlay a read-only file system like ROMFS |
| with a writable file system (like a RAM disk). This should then give |
| to a readable/write-able file system with some fixed content.</p> |
| </section> |
| <section id="prefixes"> |
| <h2>Prefixes<a class="headerlink" href="#prefixes" title="Permalink to this heading"></a></h2> |
| <p>And optional prefix may be provided with each of the file systems |
| combined in by the Union File System. For example, suppose that:</p> |
| <ul class="simple"> |
| <li><p>File system 1 is a ROMFS file system with prefix == NULL,</p></li> |
| <li><p>File system 2 is a BINFS file system with prefix == “cgin-bin”, and</p></li> |
| <li><p>The union file system is mounted at <code class="docutils literal notranslate"><span class="pre">/mnt/www</span></code>.</p></li> |
| </ul> |
| <p>Then the content in the in the ROMFS file system would appear at |
| <code class="docutils literal notranslate"><span class="pre">/mnt/www</span></code> and the content of the BINFS file system would appear at |
| <code class="docutils literal notranslate"><span class="pre">/mnt/www/cgi-gin</span></code>.</p> |
| </section> |
| <section id="example-configurations"> |
| <h2>Example Configurations<a class="headerlink" href="#example-configurations" title="Permalink to this heading"></a></h2> |
| <ul> |
| <li><p><code class="docutils literal notranslate"><span class="pre">boards/sim/sim/sim/unionfs</span></code> - This is a simulator configuration that |
| uses the Union File System test at apps/examples/unionfs. That test |
| overlays two small ROMFS file systems with many conflicts in |
| directories and file names. This is a good platform for testing the |
| Union file System and apps/examples/unionfs is a good example of how to |
| configure the Union File System.</p></li> |
| <li><p><code class="docutils literal notranslate"><span class="pre">boards/arm/lpc17xx_40xx/lincoln60/thttpd-binfs</span></code> - This is an example |
| using the THTTPD web server. It server up content from a Union File |
| System with fixed content provided by a ROMFS file system and CGI |
| content provided by a BINFS file system.</p> |
| <p>You can see how the Union File System content directory is configured |
| by logic in apps/example/thttpd/.</p> |
| </li> |
| <li><p><code class="docutils literal notranslate"><span class="pre">boards/arm/lpc17xx_40xx/olimex-lpc1766stk/thttpd-binfs</span></code> - This is |
| essentially the same as the lincoln60 configuration. It does not work, |
| however, because the LPC1766 has insufficient RAM to support the THTTPD |
| application in this configuration.</p></li> |
| </ul> |
| <p>See the documentation pages for each of these board for additional |
| information about these configurations.</p> |
| </section> |
| </section> |
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