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<h1 id="technical-behavior-and-limitations">Technical Behavior and Limitations</h1>
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<h2 id="do-hardlinks-work">Do hardlinks work?</h2>
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<p>Yes. See also the option <code>inodecalc</code> for how inode values are
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calculated.</p>
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<p>What mergerfs does not do is fake hard links across branches. Read
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the section "rename & link" for how it works.</p>
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<p>Remember that hardlinks will NOT work across devices. That includes
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between the original filesystem and a mergerfs pool, between two
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separate pools of the same underlying filesystems, or bind mounts of
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paths within the mergerfs pool. The latter is common when using Docker
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or Podman. Multiple volumes (bind mounts) to the same underlying
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filesystem are considered different devices. There is no way to link
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between them. You should mount in the highest directory in the
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mergerfs pool that includes all the paths you need if you want links
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to work.</p>
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<h2 id="how-does-mergerfs-handle-moving-and-copying-of-files">How does mergerfs handle moving and copying of files?</h2>
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<p>This is a <em>very</em> common mistaken assumption regarding how filesystems
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work. There is no such thing as "move" or "copy." These concepts are
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high level behaviors made up of numerous independent steps and <em>not</em>
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individual filesystem functions.</p>
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<p>A "move" can include a "copy" so lets describe copy first.</p>
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<p>When an application copies a file from source to destination it can do
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so in a number of ways but the basics are the following.</p>
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<ol>
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<li><code>open</code> the source file.</li>
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<li><code>create</code> the destination file.</li>
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<li><code>read</code> a chunk of data from source and <code>write</code> to
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destination. Continue till it runs out of data to copy.</li>
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<li>Copy file metadata (<code>stat</code>) such as ownership (<code>chown</code>),
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permissions (<code>chmod</code>), timestamps (<code>utimes</code>), extended attributes
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(<code>getxattr</code>, <code>setxattr</code>), etc.</li>
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<li><code>close</code> source and destination files.</li>
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</ol>
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<p>"move" is typically a <code>rename(src,dst)</code> and if that errors with
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<code>EXDEV</code> (meaning the source and destination are on different
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filesystems) the application will "copy" the file as described above
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and then it removes (<code>unlink</code>) the source.</p>
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<p>The <code>rename(src,dst)</code>, <code>open(src)</code>, <code>create(dst)</code>, data copying,
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metadata copying, <code>unlink(src)</code>, etc. are entirely distinct and
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separate events. There is really no practical way to know that what is
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ultimately occurring is the "copying" of a file or what the source
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file would be. Since the source is not known there is no way to know
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how large a created file is destined to become. This is why it is
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impossible for mergerfs to choose the branch for a <code>create</code> based on
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file size. The only context provided when a file is created, besides
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the name, is the permissions, if it is to be read and/or written, and
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some low level settings for the operating system.</p>
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<p>All of this means that mergerfs can not make decisions when a file is
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created based on file size or the source of the data. That information
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is simply not available. At best mergerfs could respond to files
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reaching a certain size when writing data or when a file is closed.</p>
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<p>Related: if a user wished to have mergerfs perform certain activities
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based on the name of a file it is common and even best practice for a
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program to write to a temporary file first and then rename to its
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final destination. That temporary file name will typically be random
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and have no indication of the type of file being written.</p>
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<h2 id="does-ficlone-or-ficlonerange-work">Does FICLONE or FICLONERANGE work?</h2>
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<p>Unfortunately not. FUSE, the technology mergerfs is based on, does not
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support the <code>clone_file_range</code> feature needed for it to work. mergerfs
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won't even know such a request is made. The kernel will simply return
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an error back to the application making the request.</p>
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<p>Should FUSE gain the ability mergerfs will be updated to support it.</p>
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<h2 id="why-do-i-get-an-out-of-space-no-space-left-on-device-enospc-error-even-though-there-appears-to-be-lots-of-space-available">Why do I get an "out of space" / "no space left on device" / ENOSPC error even though there appears to be lots of space available?</h2>
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|
<p>First make sure you've read the sections above about policies, path
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preservation, branch filtering, and the options <strong>minfreespace</strong>,
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<strong>moveonenospc</strong>, <strong>statfs</strong>, and <strong>statfs_ignore</strong>.</p>
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<p>mergerfs is simply presenting a union of the content within multiple
|
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branches. The reported free space is an aggregate of space available
|
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within the pool (behavior modified by <strong>statfs</strong> and
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<strong>statfs_ignore</strong>). It does not represent a contiguous space. In the
|
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same way that read-only filesystems, those with quotas, or reserved
|
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space report the full theoretical space available.</p>
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<p>Due to path preservation, branch tagging, read-only status, and
|
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<strong>minfreespace</strong> settings it is perfectly valid that <code>ENOSPC</code> / "out
|
|
of space" / "no space left on device" be returned. It is doing what
|
|
was asked of it: filtering possible branches due to those
|
|
settings. Only one error can be returned and if one of the reasons for
|
|
filtering a branch was <strong>minfreespace</strong> then it will be returned as
|
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such. <strong>moveonenospc</strong> is only relevant to writing a file which is too
|
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large for the filesystem it's currently on.</p>
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<p>It is also possible that the filesystem selected has run out of
|
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inodes. Use <code>df -i</code> to list the total and available inodes per
|
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filesystem.</p>
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<p>If you don't care about path preservation then simply change the
|
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<code>create</code> policy to one which isn't. <code>mfs</code> is probably what most are
|
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looking for. The reason it's not default is because it was originally
|
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set to <code>epmfs</code> and changing it now would change people's setup. Such a
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setting change will likely occur in mergerfs 3.</p>
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<h2 id="why-does-the-total-available-space-in-mergerfs-not-equal-outside">Why does the total available space in mergerfs not equal outside?</h2>
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<p>Are you using ext2/3/4? With reserve for root? mergerfs uses available
|
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space for statfs calculations. If you've reserved space for root then
|
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it won't show up.</p>
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<p>You can remove the reserve by running: <code>tune2fs -m 0 <device></code></p>
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<h2 id="i-notice-massive-slowdowns-of-writes-when-enabling-cachefiles">I notice massive slowdowns of writes when enabling cache.files.</h2>
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<p>When file caching is enabled in any form (<code>cache.files!=off</code>) it will
|
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issue <code>getxattr</code> requests for <code>security.capability</code> prior to <em>every
|
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single write</em>. This will usually result in performance degradation,
|
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especially when using a network filesystem (such as NFS or SMB.)
|
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Unfortunately at this moment, the kernel is not caching the response.</p>
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<p>To work around this situation mergerfs offers a few solutions.</p>
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<ol>
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<li>Set <code>security_capability=false</code>. It will short circuit any call and
|
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return <code>ENOATTR</code>. This still means though that mergerfs will
|
|
receive the request before every write but at least it doesn't get
|
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passed through to the underlying filesystem.</li>
|
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<li>Set <code>xattr=noattr</code>. Same as above but applies to <em>all</em> calls to
|
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getxattr. Not just <code>security.capability</code>. This will not be cached
|
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by the kernel either but mergerfs' runtime config system will still
|
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function.</li>
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<li>Set <code>xattr=nosys</code>. Results in mergerfs returning <code>ENOSYS</code> which
|
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<em>will</em> be cached by the kernel. No future xattr calls will be
|
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forwarded to mergerfs. The downside is that also means the xattr
|
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based config and query functionality won't work either.</li>
|
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<li>Disable file caching. If you aren't using applications which use
|
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<code>mmap</code> it's probably simpler to just disable it altogether. The
|
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kernel won't send the requests when caching is disabled.</li>
|
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</ol>
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<h2 id="why-cant-i-see-my-files-directories">Why can't I see my files / directories?</h2>
|
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<p>It's almost always a permissions issue. Unlike mhddfs and
|
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unionfs-fuse, which runs as root and attempts to access content as
|
|
such, mergerfs always changes its credentials to that of the
|
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caller. This means that if the user does not have access to a file or
|
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directory than neither will mergerfs. However, because mergerfs is
|
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creating a union of paths it may be able to read some files and
|
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directories on one filesystem but not another resulting in an
|
|
incomplete set.</p>
|
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<p>Whenever you run into a split permission issue (seeing some but not
|
|
all files) try using
|
|
<a href="https://github.com/trapexit/mergerfs-tools">mergerfs.fsck</a> tool to
|
|
check for and fix the mismatch. If you aren't seeing anything at all
|
|
be sure that the basic permissions are correct. The user and group
|
|
values are correct and that directories have their executable bit
|
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set. A common mistake by users new to Linux is to <code>chmod -R 644</code> when
|
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they should have <code>chmod -R u=rwX,go=rX</code>.</p>
|
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<p>If using a network filesystem such as NFS or SMB (Samba) be sure to
|
|
pay close attention to anything regarding permissioning and
|
|
users. Root squashing and user translation for instance has bitten a
|
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few mergerfs users. Some of these also affect the use of mergerfs from
|
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container platforms such as Docker.</p>
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