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<li class="md-nav__item">
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<a href="../../intro_to_filesystems/" class="md-nav__link">
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<span class="md-ellipsis">
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Intro to Filesystems
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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--nested">
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<input class="md-nav__toggle md-toggle " type="checkbox" id="__nav_6" >
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<label class="md-nav__link" for="__nav_6" id="__nav_6_label" tabindex="0">
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<span class="md-ellipsis">
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Config
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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_6_label" aria-expanded="false">
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<label class="md-nav__title" for="__nav_6">
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<span class="md-nav__icon md-icon"></span>
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Config
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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="../../config/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="../../config/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="../../config/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="../../config/branches-mount-timeout/" class="md-nav__link">
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<span class="md-ellipsis">
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branches-mount-timeout
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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="../../config/functions_categories_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="../../config/minfreespace/" class="md-nav__link">
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<span class="md-ellipsis">
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minfreespace
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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="../../config/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="../../config/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">
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<a href="../../config/moveonenospc/" class="md-nav__link">
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<span class="md-ellipsis">
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moveonenospc
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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="../../config/cache/" class="md-nav__link">
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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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</li>
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<li class="md-nav__item">
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<a href="../../config/passthrough/" class="md-nav__link">
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<span class="md-ellipsis">
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passthrough
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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="../../config/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="../../config/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="../../config/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="../../config/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="../../config/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="../../config/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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</a>
|
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</li>
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<li class="md-nav__item">
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<a href="../../config/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="../../config/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="../../config/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="../../config/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="../../config/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="../../config/xattr/" class="md-nav__link">
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<span class="md-ellipsis">
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xattr
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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="../../config/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="../../config/statfs/" class="md-nav__link">
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|
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<span class="md-ellipsis">
|
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statfs / statvfs
|
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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="../../config/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="../../config/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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<li class="md-nav__item">
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<a href="../../config/kernel-permissions-check/" class="md-nav__link">
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<span class="md-ellipsis">
|
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kernel-permissions-check
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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_and_logging/" class="md-nav__link">
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<span class="md-ellipsis">
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Error Handling and Logging
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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="../../resource_usage/" class="md-nav__link">
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<span class="md-ellipsis">
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Resource Usage and Management
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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_interface/" class="md-nav__link">
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<span class="md-ellipsis">
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Runtime Interface
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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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|
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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="../../tips_notes/" class="md-nav__link">
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<span class="md-ellipsis">
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Tips and Notes
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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="../../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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</a>
|
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</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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|
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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="../../benchmarking/" class="md-nav__link">
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<span class="md-ellipsis">
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Benchmarking
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|
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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="../../performance/" class="md-nav__link">
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<span class="md-ellipsis">
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Tweaking Performance
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|
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</span>
|
|
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</a>
|
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</li>
|
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<h1 id="technical-behavior-and-limitations">Technical Behavior and Limitations</h1>
|
|
<h2 id="do-hardlinks-work">Do hardlinks work?</h2>
|
|
<p>Yes. There is no option to enable or disable links (or renames.) They
|
|
are fundamentally supported in compatible situations. That said the
|
|
inode of a file is not necessarily indicative of two file names
|
|
linking to the same underlying data. See also the option <code>inodecalc</code>
|
|
for how inode values are calculated.</p>
|
|
<p>What mergerfs does not do is fake hard links across branches. Read the
|
|
section <a href="../../config/rename_and_link/">rename & link</a> for how it
|
|
works.</p>
|
|
<p>Remember that hardlinks will <strong>NOT</strong> work across devices. That
|
|
includes between the original filesystem and a mergerfs pool, between
|
|
two separate pools of the same underlying filesystems, or bind mounts
|
|
of paths within the mergerfs pool. The latter is common when using
|
|
Docker or Podman. Multiple volumes (bind mounts) to the same
|
|
underlying filesystem are considered different devices. There is no
|
|
way to link or rename between them. You should mount in the highest
|
|
directory in the mergerfs pool that includes all the paths you need if
|
|
you want links and rename to work.</p>
|
|
<h2 id="do-reflink-ficlone-or-ficlonerange-work">Do reflink, FICLONE, or FICLONERANGE work?</h2>
|
|
<p>Unfortunately not. FUSE, the technology mergerfs is based on, does not
|
|
support the <code>clone_file_range</code> feature needed for it to work. mergerfs
|
|
won't even know such a request is made. The kernel will simply return
|
|
an error back to the application making the request.</p>
|
|
<p>Should FUSE gain the ability mergerfs will be updated to support it.</p>
|
|
<p>That said: mergerfs itself will attempt to use <code>FICLONE</code> when copying
|
|
data between files in the limited cases where it copies/moves files.</p>
|
|
<h2 id="how-does-mergerfs-handle-moving-and-copying-of-files">How does mergerfs handle moving and copying of files?</h2>
|
|
<p>This is a <em>very</em> common mistaken assumption regarding how filesystems
|
|
work. There is no such thing as "move" or "copy." These concepts are
|
|
high level behaviors made up of numerous independent steps and <em>not</em>
|
|
individual filesystem functions.</p>
|
|
<p>A "move" can include a "copy" so lets describe copy first.</p>
|
|
<p>When an application copies a file from "source" to "destination" it
|
|
can do so in a number of ways but the basics are the following.</p>
|
|
<ol>
|
|
<li><code>open</code> the source file.</li>
|
|
<li><code>create</code> the destination file.</li>
|
|
<li><code>read</code> a chunk of data from source and <code>write</code> to
|
|
destination. Continue till it runs out of data to copy.</li>
|
|
<li>Copy file metadata (<code>stat</code>) such as ownership (<code>chown</code>),
|
|
permissions (<code>chmod</code>), timestamps (<code>utimes</code>), extended attributes
|
|
(<code>getxattr</code>, <code>setxattr</code>), etc.</li>
|
|
<li><code>close</code> source and destination files.</li>
|
|
</ol>
|
|
<p>"move" is typically a <code>rename(src,dst)</code> and if that errors with
|
|
<code>EXDEV</code> (meaning the source and destination are on different
|
|
filesystems) the application will "copy" the file as described above
|
|
and then it removes (<code>unlink</code>) the source.</p>
|
|
<p>The <code>rename(src,dst)</code>, <code>open(src)</code>, <code>create(dst)</code>, data copying,
|
|
metadata copying, <code>unlink(src)</code>, etc. are entirely distinct and
|
|
separate events. There is really no practical way to know that what is
|
|
ultimately occurring is the "copying" of a file or what the source
|
|
file would be. Since the source is not known there is no way to know
|
|
how large a created file is destined to become. This is why it is
|
|
impossible for mergerfs to choose the branch for a <code>create</code> based on
|
|
file size. The only context provided when a file is created, besides
|
|
the name, is the permissions, if it is to be read and/or written, and
|
|
some low level settings for the operating system.</p>
|
|
<p>All of this means that mergerfs can not make decisions when a file is
|
|
created based on file size or the source of the data. That information
|
|
is simply not available. At best mergerfs could respond to files
|
|
reaching a certain size when writing data, a file is closed, or
|
|
renamed.</p>
|
|
<p>Related: if a user wished to have mergerfs perform certain activities
|
|
based on the name of a file it is common and even best practice for a
|
|
program to write to a temporary file first and then rename to its
|
|
final destination. That temporary file name will typically be random
|
|
and have no indication of the type of file being written. At best
|
|
something could be done on rename.</p>
|
|
<p>Additional reading: <a href="../../intro_to_filesystems/#workflows">Intro to Filesystems:
|
|
Workflows</a></p>
|
|
<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>
|
|
<p>Are you using ext2/3/4? With reserve for root? mergerfs uses available
|
|
space for statfs calculations. If you've reserved space for root then
|
|
it won't show up.</p>
|
|
<p>You can remove the reserve by running: <code>tune2fs -m 0 <device></code></p>
|
|
<h2 id="i-notice-massive-slowdowns-of-writes-when-enabling-cachefiles">I notice massive slowdowns of writes when enabling cache.files.</h2>
|
|
<p>When file caching is enabled in any form (<code>cache.files!=off</code>) it will
|
|
issue <code>getxattr</code> requests for <code>security.capability</code> prior to <em>every
|
|
single write</em>. This will usually result in performance degradation,
|
|
especially when using a network filesystem (such as NFS or SMB.)
|
|
Unfortunately at this moment, the kernel is not caching the response.</p>
|
|
<p>To work around this situation mergerfs offers a few solutions.</p>
|
|
<ol>
|
|
<li>Set <code>security_capability=false</code>. It will short circuit any call and
|
|
return <code>ENOATTR</code>. This still means though that mergerfs will
|
|
receive the request before every write but at least it doesn't get
|
|
passed through to the underlying filesystem.</li>
|
|
<li>Set <code>xattr=noattr</code>. Same as above but applies to <em>all</em> calls to
|
|
getxattr. Not just <code>security.capability</code>. This will not be cached
|
|
by the kernel either but mergerfs' runtime config system will still
|
|
function.</li>
|
|
<li>Set <code>xattr=nosys</code>. Results in mergerfs returning <code>ENOSYS</code> which
|
|
<em>will</em> be cached by the kernel. No future xattr calls will be
|
|
forwarded to mergerfs. The downside is that also means the xattr
|
|
based config and query functionality won't work either.</li>
|
|
<li>Disable file caching. If you aren't using applications which use
|
|
<code>mmap</code> it's probably simpler to just disable it altogether. The
|
|
kernel won't send the requests when caching is disabled.</li>
|
|
</ol>
|
|
<h2 id="why-use-fuse-why-not-a-kernel-based-solution">Why use FUSE? Why not a kernel based solution?</h2>
|
|
<p>As with any solution to a problem, there are advantages and
|
|
disadvantages to each one.</p>
|
|
<p>A FUSE based solution has all the downsides of FUSE:</p>
|
|
<ul>
|
|
<li>Higher IO latency due to the trips in and out of kernel space</li>
|
|
<li>Higher general overhead due to trips in and out of kernel space</li>
|
|
<li>Double caching when using page caching</li>
|
|
<li>Misc limitations due to FUSE's design</li>
|
|
</ul>
|
|
<p>But FUSE also has a lot of upsides:</p>
|
|
<ul>
|
|
<li>Easier to offer a cross platform solution</li>
|
|
<li>Easier forward and backward compatibility</li>
|
|
<li>Easier updates for users</li>
|
|
<li>Easier and faster release cadence</li>
|
|
<li>Allows more flexibility in design and features</li>
|
|
<li>Overall easier to write, secure, and maintain</li>
|
|
<li>Much lower barrier to entry (getting code into the kernel takes a
|
|
lot of time and effort initially)</li>
|
|
</ul>
|
|
<h2 id="is-my-oss-libfuse-needed-for-mergerfs-to-work">Is my OS's libfuse needed for mergerfs to work?</h2>
|
|
<p>No. Normally <code>mount.fuse</code> is needed to get mergerfs (or any FUSE
|
|
filesystem to mount using the <code>mount</code> command but in vendoring the
|
|
libfuse library the <code>mount.fuse</code> app has been renamed to
|
|
<code>mount.mergerfs</code> meaning the filesystem type in <code>fstab</code> can simply be
|
|
<code>mergerfs</code>. That said there should be no harm in having it installed
|
|
and continuing to using <code>fuse.mergerfs</code> as the type in <code>/etc/fstab</code>.</p>
|
|
<p>If <code>mergerfs</code> doesn't work as a type it could be due to how the
|
|
<code>mount.mergerfs</code> tool was installed. Must be in <code>/sbin/</code> with proper
|
|
permissions.</p>
|
|
<h2 id="why-was-splice-support-removed">Why was splice support removed?</h2>
|
|
<p>After a lot of testing over the years, splicing always appeared to
|
|
at best, provide equivalent performance, and in some cases, worse
|
|
performance. Splice is not supported on other platforms forcing a
|
|
traditional read/write fallback to be provided. The splice code was
|
|
removed to simplify the codebase.</p>
|
|
<h2 id="how-does-mergerfs-handle-credentials">How does mergerfs handle credentials?</h2>
|
|
<p>mergerfs is a multithreaded application in order to handle requests
|
|
from the kernel concurrently. Each FUSE message has a header with
|
|
certain details about the request including the process ID (pid) of
|
|
the requesting application, the process' effective user id (uid), and
|
|
group id (gid). To ensure proper POSIX filesystem behavior and
|
|
security mergerfs must change its identity to match that of the
|
|
requester when performing the certain functions on the underlying
|
|
filesystem. As required by standards most Unix/POSIX based systems a
|
|
process and all its threads are under the same uid and gid. However,
|
|
on Linux each thread <strong>may</strong> have its own credentials. This allows
|
|
mergerfs to be multithreaded and for each thread to change to the
|
|
credentials as required by the incoming message it is
|
|
handling. However, currently on FreeBSD this is not possible (though
|
|
there has been
|
|
<a href="https://wiki.freebsd.org/Per-Thread%20Credentials">discussions</a>) and
|
|
as such must change the credentials of the whole application when
|
|
actioning messages. mergerfs does optimize this behavior by only
|
|
changing credentials and locking the thread to do so if the process is
|
|
currently not the same as what is necessary by the incoming
|
|
request. As a result of this design FreeBSD may experience more
|
|
contention and therefore lower performance than Linux.</p>
|
|
<p>Additionally, mergerfs <a href="../../known_issues_bugs/#supplemental-user-groups">utilizes a cache for supplemental
|
|
groups</a> due the the
|
|
high cost of querying that information.</p>
|
|
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