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caching
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</span>
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</a>
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</li>
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<li class="md-nav__item">
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<a href="../options/" class="md-nav__link">
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<span class="md-ellipsis">
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Options
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</span>
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</a>
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</li>
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<li class="md-nav__item">
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<a href="../deprecated_options/" class="md-nav__link">
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<span class="md-ellipsis">
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Deprecated Options
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</span>
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</a>
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</li>
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<li class="md-nav__item">
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<a href="../branches/" class="md-nav__link">
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<span class="md-ellipsis">
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branches
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</span>
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</a>
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</li>
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<li class="md-nav__item">
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<a href="../functions_categories_and_policies/" class="md-nav__link">
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<span class="md-ellipsis">
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functions, categories and policies
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</span>
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</a>
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</li>
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<li class="md-nav__item">
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<a href="../func_readdir/" class="md-nav__link">
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<span class="md-ellipsis">
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func.readdir
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</span>
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</a>
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</li>
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<li class="md-nav__item">
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<a href="../rename_and_link/" class="md-nav__link">
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<span class="md-ellipsis">
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rename and link
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</span>
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</a>
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</li>
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<li class="md-nav__item md-nav__item--active">
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<input class="md-nav__toggle md-toggle" type="checkbox" id="__toc">
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<label class="md-nav__link md-nav__link--active" for="__toc">
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<span class="md-ellipsis">
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caching
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</span>
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<span class="md-nav__icon md-icon"></span>
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</label>
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<a href="./" class="md-nav__link md-nav__link--active">
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<span class="md-ellipsis">
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caching
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</span>
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</a>
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<nav class="md-nav md-nav--secondary" aria-label="Table of contents">
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<label class="md-nav__title" for="__toc">
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<span class="md-nav__icon md-icon"></span>
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Table of contents
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</label>
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<ul class="md-nav__list" data-md-component="toc" data-md-scrollfix>
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<li class="md-nav__item">
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<a href="#cachefiles" class="md-nav__link">
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<span class="md-ellipsis">
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cache.files
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</span>
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</a>
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</li>
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<li class="md-nav__item">
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<a href="#cacheentry" class="md-nav__link">
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<span class="md-ellipsis">
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cache.entry
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</span>
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</a>
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</li>
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<li class="md-nav__item">
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<a href="#cachenegative_entry" class="md-nav__link">
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<span class="md-ellipsis">
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cache.negative_entry
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</span>
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</a>
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</li>
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<li class="md-nav__item">
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<a href="#cacheattr" class="md-nav__link">
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<span class="md-ellipsis">
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cache.attr
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</span>
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</a>
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</li>
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<li class="md-nav__item">
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<a href="#cachestatfs" class="md-nav__link">
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<span class="md-ellipsis">
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cache.statfs
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</span>
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</a>
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</li>
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<li class="md-nav__item">
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<a href="#cachesymlinks" class="md-nav__link">
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<span class="md-ellipsis">
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cache.symlinks
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</span>
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</a>
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</li>
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<li class="md-nav__item">
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<a href="#cachereaddir" class="md-nav__link">
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<span class="md-ellipsis">
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cache.readdir
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</span>
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</a>
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</li>
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<li class="md-nav__item">
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<a href="#cachewriteback" class="md-nav__link">
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<span class="md-ellipsis">
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cache.writeback
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</span>
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</a>
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</li>
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</ul>
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</nav>
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</li>
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<li class="md-nav__item">
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<a href="../readahead/" class="md-nav__link">
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<span class="md-ellipsis">
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readahead
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</span>
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</a>
|
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</li>
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<li class="md-nav__item">
|
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<a href="../inodecalc/" class="md-nav__link">
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<span class="md-ellipsis">
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inodecalc
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</span>
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</a>
|
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</li>
|
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<li class="md-nav__item">
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<a href="../threads/" class="md-nav__link">
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<span class="md-ellipsis">
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threads
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</span>
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</a>
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</li>
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<li class="md-nav__item">
|
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<a href="../pin-threads/" class="md-nav__link">
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<span class="md-ellipsis">
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pin-threads
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</span>
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</a>
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</li>
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<li class="md-nav__item">
|
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<a href="../link_cow/" class="md-nav__link">
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<span class="md-ellipsis">
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link_cow
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</span>
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</a>
|
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</li>
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<li class="md-nav__item">
|
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<a href="../fuse_msg_size/" class="md-nav__link">
|
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<span class="md-ellipsis">
|
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fuse_msg_size
|
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</span>
|
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|
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</a>
|
|
</li>
|
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<li class="md-nav__item">
|
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<a href="../follow-symlinks/" class="md-nav__link">
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<span class="md-ellipsis">
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follow-symlinks
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</span>
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</a>
|
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</li>
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<li class="md-nav__item">
|
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<a href="../link-exdev/" class="md-nav__link">
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<span class="md-ellipsis">
|
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link-exdev
|
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</span>
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</a>
|
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</li>
|
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<li class="md-nav__item">
|
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<a href="../rename-exdev/" class="md-nav__link">
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<span class="md-ellipsis">
|
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rename-exdev
|
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</span>
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</a>
|
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</li>
|
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<li class="md-nav__item">
|
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<a href="../symlinkify/" class="md-nav__link">
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<span class="md-ellipsis">
|
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symlinkify
|
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</span>
|
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</a>
|
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</li>
|
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<li class="md-nav__item">
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<a href="../nullrw/" class="md-nav__link">
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<span class="md-ellipsis">
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nullrw
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</span>
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</a>
|
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</li>
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<li class="md-nav__item">
|
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<a href="../xattr/" class="md-nav__link">
|
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<span class="md-ellipsis">
|
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xattr
|
|
</span>
|
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</a>
|
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</li>
|
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<li class="md-nav__item">
|
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<a href="../nfsopenhack/" class="md-nav__link">
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<span class="md-ellipsis">
|
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nfsopenhack
|
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</span>
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</a>
|
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</li>
|
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<li class="md-nav__item">
|
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<a href="../statfs/" class="md-nav__link">
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<span class="md-ellipsis">
|
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statfs / statvfs
|
|
</span>
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</a>
|
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</li>
|
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<li class="md-nav__item">
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<a href="../flush-on-close/" class="md-nav__link">
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<span class="md-ellipsis">
|
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flush-on-close
|
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</span>
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</a>
|
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</li>
|
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<li class="md-nav__item">
|
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<a href="../export-support/" class="md-nav__link">
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<span class="md-ellipsis">
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export-support
|
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</span>
|
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</a>
|
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</li>
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</ul>
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</nav>
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</li>
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<li class="md-nav__item">
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<a href="../../error_handling/" class="md-nav__link">
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<span class="md-ellipsis">
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Error Handling
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</span>
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</a>
|
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</li>
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<li class="md-nav__item">
|
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<a href="../../runtime_interfaces/" class="md-nav__link">
|
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<span class="md-ellipsis">
|
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Runtime Interfaces
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</span>
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</a>
|
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</li>
|
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<li class="md-nav__item">
|
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<a href="../../remote_filesystems/" class="md-nav__link">
|
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<span class="md-ellipsis">
|
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Remote Filesystems
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</span>
|
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|
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</a>
|
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</li>
|
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<li class="md-nav__item">
|
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<a href="../../tips_notes/" class="md-nav__link">
|
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|
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<span class="md-ellipsis">
|
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Tips and Notes
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</span>
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|
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</a>
|
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</li>
|
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<li class="md-nav__item">
|
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<a href="../../known_issues_bugs/" class="md-nav__link">
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<span class="md-ellipsis">
|
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Known Issues and Bugs
|
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</span>
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|
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</a>
|
|
</li>
|
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<li class="md-nav__item">
|
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<a href="../../project_comparisons/" class="md-nav__link">
|
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<span class="md-ellipsis">
|
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Project Comparisons
|
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</span>
|
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|
|
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</a>
|
|
</li>
|
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<li class="md-nav__item">
|
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<a href="../../performance/" class="md-nav__link">
|
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|
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|
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<span class="md-ellipsis">
|
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Tweaking Performance
|
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</span>
|
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|
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|
|
</a>
|
|
</li>
|
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<li class="md-nav__item">
|
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<a href="../../benchmarking/" class="md-nav__link">
|
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|
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|
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<span class="md-ellipsis">
|
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Benchmarking
|
|
</span>
|
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|
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</a>
|
|
</li>
|
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<li class="md-nav__item">
|
|
<a href="../../tooling/" class="md-nav__link">
|
|
|
|
|
|
<span class="md-ellipsis">
|
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Tooling
|
|
</span>
|
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|
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</a>
|
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</li>
|
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<li class="md-nav__item md-nav__item--nested">
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<input class="md-nav__toggle md-toggle " type="checkbox" id="__nav_14" >
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<label class="md-nav__link" for="__nav_14" id="__nav_14_label" tabindex="0">
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<span class="md-ellipsis">
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FAQ
|
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</span>
|
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<span class="md-nav__icon md-icon"></span>
|
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</label>
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<nav class="md-nav" data-md-level="1" aria-labelledby="__nav_14_label" aria-expanded="false">
|
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<label class="md-nav__title" for="__nav_14">
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<span class="md-nav__icon md-icon"></span>
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FAQ
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</label>
|
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<ul class="md-nav__list" data-md-scrollfix>
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<li class="md-nav__item">
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<a href="../../faq/reliability_and_scalability/" class="md-nav__link">
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<span class="md-ellipsis">
|
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Reliability and Scalability
|
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</span>
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|
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</a>
|
|
</li>
|
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<li class="md-nav__item">
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cache.files
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cache.statfs
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cache.symlinks
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cache.readdir
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cache.writeback
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<div class="md-content" data-md-component="content">
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<article class="md-content__inner md-typeset">
|
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<a href="https://github.com/trapexit/mergerfs/tree/master/mkdocs/docs/config/cache.md" title="Edit this page" class="md-content__button md-icon">
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</a>
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|
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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.</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>
|
|
<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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|
|
|
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</article>
|
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