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<!doctype html><html lang="en" class="no-js"> <head> <meta charset="utf-8"> <meta name="viewport" content="width=device-width,initial-scale=1"> <meta name="description" content="mergerfs - a featureful union filesystem"> <link rel="canonical" href="https://trapexit.github.io/mergerfs/config/readahead/"> <link rel="prev" href="../cache/"> <link rel="next" href="../inodecalc/"> <link rel="icon" href="../../assets/images/favicon.png"> <meta name="generator" content="mkdocs-1.6.1, mkdocs-material-9.5.49"> <title>readahead - mergerfs</title> <link rel="stylesheet" href="../../assets/stylesheets/main.6f8fc17f.min.css"> <link rel="stylesheet" href="../../assets/stylesheets/palette.06af60db.min.css">
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<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>
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<li class="md-nav__item"> <a href="../threads/" class="md-nav__link"> <span class="md-ellipsis"> threads </span>
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<li class="md-nav__item"> <a href="../pin-threads/" class="md-nav__link"> <span class="md-ellipsis"> pin-threads </span>
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<li class="md-nav__item"> <a href="../fuse_msg_size/" class="md-nav__link"> <span class="md-ellipsis"> fuse_msg_size </span>
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<li class="md-nav__item"> <a href="../link-exdev/" class="md-nav__link"> <span class="md-ellipsis"> link-exdev </span>
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<li class="md-nav__item"> <a href="../rename-exdev/" class="md-nav__link"> <span class="md-ellipsis"> rename-exdev </span>
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<li class="md-nav__item"> <a href="../nullrw/" class="md-nav__link"> <span class="md-ellipsis"> nullrw </span>
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<li class="md-nav__item"> <a href="../xattr/" class="md-nav__link"> <span class="md-ellipsis"> xattr </span>
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<li class="md-nav__item"> <a href="../nfsopenhack/" class="md-nav__link"> <span class="md-ellipsis"> nfsopenhack </span>
</a> </li>
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</a> </li>
<li class="md-nav__item"> <a href="../flush-on-close/" class="md-nav__link"> <span class="md-ellipsis"> flush-on-close </span>
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<li class="md-nav__item"> <a href="../../remote_filesystems/" class="md-nav__link"> <span class="md-ellipsis"> Remote Filesystems </span>
</a> </li>
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<li class="md-nav__item"> <a href="../../faq/limit_drive_spinup/" class="md-nav__link"> <span class="md-ellipsis"> Limiting drive spinup </span>
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<h1 id="readahead">readahead</h1><p>Sets the mergerfs and underlying filesystem <code>readahead</code> values. Thevalue unit is in kibibytes.</p><ul><li><code>readahead=1024</code></li></ul><p>While the max size of messages sent between the kernel and mergerfs isconfigurable via the <a href="../fuse_msg_size/">fuse_msg_size</a> option thatdoesn't mean that is the size used by the kernel for read andwrites.</p><p>Linux has a max read/write size of 2GB. Since the max FUSE messagesize is just over 1MB the kernel will break up read and write requestswith buffers larger than that 1MB.</p><p>When page caching is disabled (<code>cache.files=off</code>), besides the kernelbreaking up requests with larger buffers, requests are effectively onefor one to mergerfs. A read or write request for X bytes is made tothe kernel and a request for X bytes is made to mergerfs. No<a href="https://en.wikipedia.org/wiki/Readahead">readahead</a> behavior willoccur because there is no page cache available for it to store thatdata. In FUSE this is referred to as "direct IO". Note that "directIO" is not the same as <code>O_DIRECT</code>.</p><p>When page caching is enabled the kernel can and will utilize<code>readahead</code>. However, there are two values which impact the size ofthe <code>readahead</code> requests. The filesystem's <code>readahead</code> value and theFUSE <code>max_readahead</code> value. Whichever is lowest is used. The default<code>max_readahead</code> in mergerfs is maxed out meaning only the filesystem<code>readahead</code> value is relevant.</p><p>Preferably this value would be set by the user externally since it isa generic feature but there is no standard way to do so mergerfs addedthis feature to make it easier to set.</p><p>There is currently no way to set separate values for differentbranches through mergerfs.</p>
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