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Create tag ${ searchTerm }
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mergerfs/preview/runtime_interface/index.html

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QuickStart
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Installation
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Installation
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Upgrade
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Build
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Terminology
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Intro to Filesystems
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Config
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Config
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Options
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Deprecated Options
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branches
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branches-mount-timeout
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functions, categories and policies
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minfreespace
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func.readdir
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<a href="../config/rename_and_link/" class="md-nav__link">
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rename and link
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moveonenospc
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<a href="../config/cache/" class="md-nav__link">
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caching
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passthrough
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<a href="../config/readahead/" class="md-nav__link">
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readahead
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<a href="../config/inodecalc/" class="md-nav__link">
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inodecalc
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<a href="../config/threads/" class="md-nav__link">
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threads
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pin-threads
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link_cow
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<a href="../config/fuse_msg_size/" class="md-nav__link">
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fuse_msg_size
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follow-symlinks
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link-exdev
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rename-exdev
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symlinkify
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nullrw
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<a href="../config/xattr/" class="md-nav__link">
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xattr
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</a>
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<a href="../config/nfsopenhack/" class="md-nav__link">
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nfsopenhack
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</a>
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<a href="../config/statfs/" class="md-nav__link">
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statfs / statvfs
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flush-on-close
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<a href="../config/export-support/" class="md-nav__link">
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export-support
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<a href="../config/kernel-permissions-check/" class="md-nav__link">
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kernel-permissions-check
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Error Handling and Logging
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Resource Usage and Management
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Runtime Interface
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Runtime Interface
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Table of contents
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<h1 id="runtime-interface">Runtime Interface</h1>
<p><code>mergerfs</code> has runtime interfaces allowing users to query certain
filesystem information, get and set config, and trigger certain
activities while it is running.</p>
<p>The interface is provided via the POSIX extended attributes filesystem
API which is a namespaced <code>key=value</code> pair store associated with a
file. Since <code>mergerfs</code> primarily uses <code>key=value</code> pairs for config it
fits well and is a known and reasonably well supported API.</p>
<p>There are two targets for <code>xattr</code> calls. One is a pseudo file used for
getting and setting config and issuing certain commands. The other are
files found on the filesystem for querying certain <code>mergerfs</code> specific
information about them.</p>
<h2 id="mergerfs-pseudo-file">.mergerfs pseudo file</h2>
<div class="language-text highlight"><pre><span></span><code><span id="__span-0-1"><a id="__codelineno-0-1" name="__codelineno-0-1" href="#__codelineno-0-1"></a>&lt;mountpoint&gt;/.mergerfs
</span></code></pre></div>
<p><code>mergerfs</code> provides this pseudo file for the runtime modification of
certain options and issuing commands. The file will not show up in
<code>readdir</code> but can be <code>stat</code>'ed and viewed/manipulated via
<a href="http://linux.die.net/man/2/listxattr">listxattr</a>,
<a href="https://linux.die.net/man/2/getxattr">getxattr</a>, and
<a href="https://linux.die.net/man/2/setxattr">setxattr</a> calls.</p>
<p>Any changes made at runtime are <strong>NOT</strong> persisted. If you wish for
values to persist they must be included as options wherever you
configure the mounting of mergerfs (/etc/fstab, systemd, etc.).</p>
<h3 id="command-line-tooling">Command Line Tooling</h3>
<p>Extended attributes is prevelant enough that there are common tools
available for interacting with them.</p>
<p>In Debian / Ubuntu distributions you can get the tools
<a href="https://linux.die.net/man/1/getfattr">getfattr</a> and
<a href="https://linux.die.net/man/1/setfattr">setfattr</a> from
<a href="https://linux.die.net/man/5/attr">attr</a> package.</p>
<div class="language-text highlight"><pre><span></span><code><span id="__span-1-1"><a id="__codelineno-1-1" name="__codelineno-1-1" href="#__codelineno-1-1"></a>$ sudo apt-get install attr
</span></code></pre></div>
<h3 id="config">Config</h3>
<h4 id="keys">Keys</h4>
<p>Use <code>getfattr -d /mountpoint/.mergerfs</code> to see all supported
configuration keys. It is effectively the same as the
<a href="../config/options/">options</a> prefixed with <code>user.mergerfs.</code>. Some are
informational or only can be set at startup and therefore read-only.</p>
<p>Example: option <code>cache.files</code> would be <code>user.mergerfs.cache.files</code>.</p>
<h4 id="values">Values</h4>
<p>Same as the <a href="../config/options/">command line options</a>.</p>
<h4 id="getting">Getting</h4>
<p><code>getfattr -n user.mergerfs.branches /mountpoint/.mergerfs</code></p>
<p><code>ENOATTR</code> will be returned if the key doesn't exist as normal with
<a href="https://linux.die.net/man/2/getxattr">getxattr</a>.</p>
<h4 id="setting">Setting</h4>
<p><code>setfattr -n user.mergerfs.branches -v VALUE /mountpoint/.mergerfs</code></p>
<p><a href="https://linux.die.net/man/2/setxattr">setxattr</a> will return <code>EROFS</code>
(Read-only filesystem) on read-only keys. <code>ENOATTR</code> will be returned
if the key does not exist. If the value attempting to be set is not
valid <code>EINVAL</code> will be returned.</p>
<h4 id="usermergerfsbranches">user.mergerfs.branches</h4>
<p><code>branches</code> has the ability to understand some simple instructions to
make manipulation of the list easier. The <code>[list]</code> is simply what is
described in the <a href="../config/branches/">branches</a> docs.</p>
<table>
<thead>
<tr>
<th>Value</th>
<th>Description</th>
</tr>
</thead>
<tbody>
<tr>
<td>[list]</td>
<td>set</td>
</tr>
<tr>
<td>+&lt;[list]</td>
<td>prepend to existing list</td>
</tr>
<tr>
<td>+&gt;[list]</td>
<td>append to existing list</td>
</tr>
<tr>
<td>-[list]</td>
<td>remove all values provided</td>
</tr>
<tr>
<td>-&lt;</td>
<td>remove first in list</td>
</tr>
<tr>
<td>-&gt;</td>
<td>remove last in list</td>
</tr>
</tbody>
</table>
<p><strong>NOTE:</strong> if the value of <code>branches</code> is set to something invalid /
non-existant <code>mergerfs</code> will return a bogus entry when the mount point
directory is <code>stat</code>'ed and create a fake file entry when listing the
directory telling the user "error: no valid mergerfs branch found,
check config". This is done to ensure the user understands the
situation and continue to be able to access the xattr interface.</p>
<h4 id="example">Example</h4>
<div class="language-text highlight"><pre><span></span><code><span id="__span-2-1"><a id="__codelineno-2-1" name="__codelineno-2-1" href="#__codelineno-2-1"></a>[trapexit:/mnt/mergerfs] $ getfattr -d .mergerfs
</span><span id="__span-2-2"><a id="__codelineno-2-2" name="__codelineno-2-2" href="#__codelineno-2-2"></a>user.mergerfs.branches=&quot;/mnt/a=RW:/mnt/b=RW&quot;
</span><span id="__span-2-3"><a id="__codelineno-2-3" name="__codelineno-2-3" href="#__codelineno-2-3"></a>user.mergerfs.minfreespace=&quot;4294967295&quot;
</span><span id="__span-2-4"><a id="__codelineno-2-4" name="__codelineno-2-4" href="#__codelineno-2-4"></a>user.mergerfs.moveonenospc=&quot;false&quot;
</span><span id="__span-2-5"><a id="__codelineno-2-5" name="__codelineno-2-5" href="#__codelineno-2-5"></a>...
</span><span id="__span-2-6"><a id="__codelineno-2-6" name="__codelineno-2-6" href="#__codelineno-2-6"></a>
</span><span id="__span-2-7"><a id="__codelineno-2-7" name="__codelineno-2-7" href="#__codelineno-2-7"></a>[trapexit:/mnt/mergerfs] $ getfattr -n user.mergerfs.category.create .mergerfs
</span><span id="__span-2-8"><a id="__codelineno-2-8" name="__codelineno-2-8" href="#__codelineno-2-8"></a>user.mergerfs.category.search=&quot;mfs&quot;
</span><span id="__span-2-9"><a id="__codelineno-2-9" name="__codelineno-2-9" href="#__codelineno-2-9"></a>
</span><span id="__span-2-10"><a id="__codelineno-2-10" name="__codelineno-2-10" href="#__codelineno-2-10"></a>[trapexit:/mnt/mergerfs] $ setfattr -n user.mergerfs.category.create -v pfrd .mergerfs
</span><span id="__span-2-11"><a id="__codelineno-2-11" name="__codelineno-2-11" href="#__codelineno-2-11"></a>[trapexit:/mnt/mergerfs] $ getfattr -n user.mergerfs.category.create .mergerfs
</span><span id="__span-2-12"><a id="__codelineno-2-12" name="__codelineno-2-12" href="#__codelineno-2-12"></a>user.mergerfs.category.search=&quot;prfd&quot;
</span><span id="__span-2-13"><a id="__codelineno-2-13" name="__codelineno-2-13" href="#__codelineno-2-13"></a>
</span><span id="__span-2-14"><a id="__codelineno-2-14" name="__codelineno-2-14" href="#__codelineno-2-14"></a>[trapexit:/mnt/mergerfs] $ setfattr -n user.mergerfs.branches -v &quot;&#39;+&lt;/mnt/c=RO .mergerfs
</span><span id="__span-2-15"><a id="__codelineno-2-15" name="__codelineno-2-15" href="#__codelineno-2-15"></a>[trapexit:/mnt/mergerfs] $ getfattr -n user.mergerfs.branches .mergerfs
</span><span id="__span-2-16"><a id="__codelineno-2-16" name="__codelineno-2-16" href="#__codelineno-2-16"></a>user.mergerfs.branches=&quot;/mnt/c=RO:/mnt/a=RW:/mnt/b=RW&quot;
</span></code></pre></div>
<h3 id="commands">Commands</h3>
<p>There are a number of commands / behaviors which can be triggerd by
writing (<a href="https://linux.die.net/man/1/setfattr">setfattr</a>,
<a href="https://linux.die.net/man/2/setxattr">setxattr</a>) particular xattr
keys of <code>/mountpoint/.mergerfs</code>. These keys do not show up in key
listings.</p>
<p>Commands can take an argument however currently no command uses or
allows a value.</p>
<table>
<thead>
<tr>
<th>Key</th>
<th>Value</th>
<th>Description</th>
</tr>
</thead>
<tbody>
<tr>
<td>user.mergerfs.cmd.gc</td>
<td>(empty)</td>
<td>Trigger a thorough garbage collection of certain pools of resources. The xattr value is not used.</td>
</tr>
<tr>
<td>user.mergerfs.cmd.gc1</td>
<td>(empty)</td>
<td>Trigger a simple garbage collection of certain pools of resources. This is also done on a timer.</td>
</tr>
<tr>
<td>user.mergerfs.cmd.invalidate-all-nodes</td>
<td>(empty)</td>
<td>Attempts to invalidate FUSE file nodes. Primarily used for debugging.</td>
</tr>
<tr>
<td>user.mergerfs.cmd.invalidate-gid-cache</td>
<td>(empty)</td>
<td>Invalidates all entries in gid cache. Primarily used for debugging but can also be useful in cases where gid supplemental groups change.</td>
</tr>
<tr>
<td>user.mergerfs.cmd.clear-gid-cache</td>
<td>(empty)</td>
<td>Clears all entries in the gid cache. Primarily used for debugging but can also be useful in cases where gid supplemental groups change.</td>
</tr>
</tbody>
</table>
<div class="language-text highlight"><pre><span></span><code><span id="__span-3-1"><a id="__codelineno-3-1" name="__codelineno-3-1" href="#__codelineno-3-1"></a>[trapexit:/mnt/mergerfs] $ setfattr -n user.mergerfs.cmd.gc /mnt/mergerfs/.mergerfs
</span><span id="__span-3-2"><a id="__codelineno-3-2" name="__codelineno-3-2" href="#__codelineno-3-2"></a>[trapexit:/mnt/mergerfs] $ journalctl -t mergerfs | tail -n1
</span><span id="__span-3-3"><a id="__codelineno-3-3" name="__codelineno-3-3" href="#__codelineno-3-3"></a>Jul 20 15:36:18 hostname mergerfs[1931348]: running basic garbage collection
</span></code></pre></div>
<h3 id="file-directory-xattrs">file / directory xattrs</h3>
<p>There is certain information <code>mergerfs</code> knows or calculates about a
file that can be useful in building tooling or debugging.</p>
<p>The keys below can be queried (<code>getfattr -n key</code>,
<a href="http://linux.die.net/man/2/getxattr">getxattr</a>) to get the described
information. These keys will not show up in any listing (<code>getfattr
-d</code>, <a href="https://linux.die.net/man/2/listxattr">listxattr</a>). Attempting
to set them will result in an error.</p>
<table>
<thead>
<tr>
<th>Key</th>
<th>Return Value</th>
</tr>
</thead>
<tbody>
<tr>
<td>user.mergerfs.basepath</td>
<td>The base mount point for the file given the current <code>getattr</code> policy.</td>
</tr>
<tr>
<td>user.mergerfs.relpath</td>
<td>The relative path of the file from the perspective of the mount point.</td>
</tr>
<tr>
<td>user.mergerfs.fullpath</td>
<td>The full path of the original file given the <code>getattr</code> policy.</td>
</tr>
<tr>
<td>user.mergerfs.allpaths</td>
<td>A NULL ('\0') separated list of full paths to all files found.</td>
</tr>
</tbody>
</table>
<div class="language-text highlight"><pre><span></span><code><span id="__span-4-1"><a id="__codelineno-4-1" name="__codelineno-4-1" href="#__codelineno-4-1"></a>[trapexit:/mnt/mergerfs] $ getfattr -n user.mergerfs.fullpath test
</span><span id="__span-4-2"><a id="__codelineno-4-2" name="__codelineno-4-2" href="#__codelineno-4-2"></a>user.mergerfs.fullpath=&quot;/mnt/a/test&quot;
</span><span id="__span-4-3"><a id="__codelineno-4-3" name="__codelineno-4-3" href="#__codelineno-4-3"></a>[trapexit:/mnt/mergerfs] $ getfattr -n user.mergerfs.allpaths .
</span><span id="__span-4-4"><a id="__codelineno-4-4" name="__codelineno-4-4" href="#__codelineno-4-4"></a>user.mergerfs.allpaths=&quot;/mnt/a\000/mnt/b\000/mnt/c&quot;
</span></code></pre></div>
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