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  1. <!doctype html>
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  6. <meta name="description" content="mergerfs - a featureful union filesystem">
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  12. <title>Technical Behavior and Limitations - mergerfs</title>
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  26. Skip to content
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  29. <div data-md-component="announce">
  30. </div>
  31. <header class="md-header md-header--shadow" data-md-component="header">
  32. <nav class="md-header__inner md-grid" aria-label="Header">
  33. <a href="../.." title="mergerfs" class="md-header__button md-logo" aria-label="mergerfs" data-md-component="logo">
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  40. <div class="md-header__ellipsis">
  41. <div class="md-header__topic">
  42. <span class="md-ellipsis">
  43. mergerfs
  44. </span>
  45. </div>
  46. <div class="md-header__topic" data-md-component="header-topic">
  47. <span class="md-ellipsis">
  48. Technical Behavior and Limitations
  49. </span>
  50. </div>
  51. </div>
  52. </div>
  53. <form class="md-header__option" data-md-component="palette">
  54. <input class="md-option" data-md-color-media="(prefers-color-scheme)" data-md-color-scheme="default" data-md-color-primary="indigo" data-md-color-accent="indigo" aria-label="Switch to light mode" type="radio" name="__palette" id="__palette_0">
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  62. <input class="md-option" data-md-color-media="(prefers-color-scheme: dark)" data-md-color-scheme="slate" data-md-color-primary="indigo" data-md-color-accent="indigo" aria-label="Switch to system preference" type="radio" name="__palette" id="__palette_2">
  63. <label class="md-header__button md-icon" title="Switch to system preference" for="__palette_0" hidden>
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  65. </label>
  66. </form>
  67. <script>var palette=__md_get("__palette");if(palette&&palette.color){if("(prefers-color-scheme)"===palette.color.media){var media=matchMedia("(prefers-color-scheme: light)"),input=document.querySelector(media.matches?"[data-md-color-media='(prefers-color-scheme: light)']":"[data-md-color-media='(prefers-color-scheme: dark)']");palette.color.media=input.getAttribute("data-md-color-media"),palette.color.scheme=input.getAttribute("data-md-color-scheme"),palette.color.primary=input.getAttribute("data-md-color-primary"),palette.color.accent=input.getAttribute("data-md-color-accent")}for(var[key,value]of Object.entries(palette.color))document.body.setAttribute("data-md-color-"+key,value)}</script>
  68. <label class="md-header__button md-icon" for="__search">
  69. <svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24"><path d="M9.5 3A6.5 6.5 0 0 1 16 9.5c0 1.61-.59 3.09-1.56 4.23l.27.27h.79l5 5-1.5 1.5-5-5v-.79l-.27-.27A6.52 6.52 0 0 1 9.5 16 6.5 6.5 0 0 1 3 9.5 6.5 6.5 0 0 1 9.5 3m0 2C7 5 5 7 5 9.5S7 14 9.5 14 14 12 14 9.5 12 5 9.5 5"/></svg>
  70. </label>
  71. <div class="md-search" data-md-component="search" role="dialog">
  72. <label class="md-search__overlay" for="__search"></label>
  73. <div class="md-search__inner" role="search">
  74. <form class="md-search__form" name="search">
  75. <input type="text" class="md-search__input" name="query" aria-label="Search" placeholder="Search" autocapitalize="off" autocorrect="off" autocomplete="off" spellcheck="false" data-md-component="search-query" required>
  76. <label class="md-search__icon md-icon" for="__search">
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  79. </label>
  80. <nav class="md-search__options" aria-label="Search">
  81. <a href="javascript:void(0)" class="md-search__icon md-icon" title="Share" aria-label="Share" data-clipboard data-clipboard-text="" data-md-component="search-share" tabindex="-1">
  82. <svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24"><path d="M18 16.08c-.76 0-1.44.3-1.96.77L8.91 12.7c.05-.23.09-.46.09-.7s-.04-.47-.09-.7l7.05-4.11c.54.5 1.25.81 2.04.81a3 3 0 0 0 3-3 3 3 0 0 0-3-3 3 3 0 0 0-3 3c0 .24.04.47.09.7L8.04 9.81C7.5 9.31 6.79 9 6 9a3 3 0 0 0-3 3 3 3 0 0 0 3 3c.79 0 1.5-.31 2.04-.81l7.12 4.15c-.05.21-.08.43-.08.66 0 1.61 1.31 2.91 2.92 2.91s2.92-1.3 2.92-2.91A2.92 2.92 0 0 0 18 16.08"/></svg>
  83. </a>
  84. <button type="reset" class="md-search__icon md-icon" title="Clear" aria-label="Clear" tabindex="-1">
  85. <svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24"><path d="M19 6.41 17.59 5 12 10.59 6.41 5 5 6.41 10.59 12 5 17.59 6.41 19 12 13.41 17.59 19 19 17.59 13.41 12z"/></svg>
  86. </button>
  87. </nav>
  88. </form>
  89. <div class="md-search__output">
  90. <div class="md-search__scrollwrap" tabindex="0" data-md-scrollfix>
  91. <div class="md-search-result" data-md-component="search-result">
  92. <div class="md-search-result__meta">
  93. Initializing search
  94. </div>
  95. <ol class="md-search-result__list" role="presentation"></ol>
  96. </div>
  97. </div>
  98. </div>
  99. </div>
  100. </div>
  101. <div class="md-header__source">
  102. <a href="https://github.com/trapexit/mergerfs" title="Go to repository" class="md-source" data-md-component="source">
  103. <div class="md-source__icon md-icon">
  104. <svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 448 512"><!--! Font Awesome Free 6.7.2 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2024 Fonticons, Inc.--><path d="M439.55 236.05 244 40.45a28.87 28.87 0 0 0-40.81 0l-40.66 40.63 51.52 51.52c27.06-9.14 52.68 16.77 43.39 43.68l49.66 49.66c34.23-11.8 61.18 31 35.47 56.69-26.49 26.49-70.21-2.87-56-37.34L240.22 199v121.85c25.3 12.54 22.26 41.85 9.08 55a34.34 34.34 0 0 1-48.55 0c-17.57-17.6-11.07-46.91 11.25-56v-123c-20.8-8.51-24.6-30.74-18.64-45L142.57 101 8.45 235.14a28.86 28.86 0 0 0 0 40.81l195.61 195.6a28.86 28.86 0 0 0 40.8 0l194.69-194.69a28.86 28.86 0 0 0 0-40.81"/></svg>
  105. </div>
  106. <div class="md-source__repository">
  107. mergerfs
  108. </div>
  109. </a>
  110. </div>
  111. </nav>
  112. </header>
  113. <div class="md-container" data-md-component="container">
  114. <main class="md-main" data-md-component="main">
  115. <div class="md-main__inner md-grid">
  116. <div class="md-sidebar md-sidebar--primary" data-md-component="sidebar" data-md-type="navigation" >
  117. <div class="md-sidebar__scrollwrap">
  118. <div class="md-sidebar__inner">
  119. <nav class="md-nav md-nav--primary" aria-label="Navigation" data-md-level="0">
  120. <label class="md-nav__title" for="__drawer">
  121. <a href="../.." title="mergerfs" class="md-nav__button md-logo" aria-label="mergerfs" data-md-component="logo">
  122. <img src="../../logo.jpeg" alt="logo">
  123. </a>
  124. mergerfs
  125. </label>
  126. <div class="md-nav__source">
  127. <a href="https://github.com/trapexit/mergerfs" title="Go to repository" class="md-source" data-md-component="source">
  128. <div class="md-source__icon md-icon">
  129. <svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 448 512"><!--! Font Awesome Free 6.7.2 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2024 Fonticons, Inc.--><path d="M439.55 236.05 244 40.45a28.87 28.87 0 0 0-40.81 0l-40.66 40.63 51.52 51.52c27.06-9.14 52.68 16.77 43.39 43.68l49.66 49.66c34.23-11.8 61.18 31 35.47 56.69-26.49 26.49-70.21-2.87-56-37.34L240.22 199v121.85c25.3 12.54 22.26 41.85 9.08 55a34.34 34.34 0 0 1-48.55 0c-17.57-17.6-11.07-46.91 11.25-56v-123c-20.8-8.51-24.6-30.74-18.64-45L142.57 101 8.45 235.14a28.86 28.86 0 0 0 0 40.81l195.61 195.6a28.86 28.86 0 0 0 40.8 0l194.69-194.69a28.86 28.86 0 0 0 0-40.81"/></svg>
  130. </div>
  131. <div class="md-source__repository">
  132. mergerfs
  133. </div>
  134. </a>
  135. </div>
  136. <ul class="md-nav__list" data-md-scrollfix>
  137. <li class="md-nav__item">
  138. <a href="../.." class="md-nav__link">
  139. <span class="md-ellipsis">
  140. Home
  141. </span>
  142. </a>
  143. </li>
  144. <li class="md-nav__item">
  145. <a href="../../quickstart/" class="md-nav__link">
  146. <span class="md-ellipsis">
  147. QuickStart
  148. </span>
  149. </a>
  150. </li>
  151. <li class="md-nav__item md-nav__item--nested">
  152. <input class="md-nav__toggle md-toggle " type="checkbox" id="__nav_3" >
  153. <label class="md-nav__link" for="__nav_3" id="__nav_3_label" tabindex="0">
  154. <span class="md-ellipsis">
  155. Installation
  156. </span>
  157. <span class="md-nav__icon md-icon"></span>
  158. </label>
  159. <nav class="md-nav" data-md-level="1" aria-labelledby="__nav_3_label" aria-expanded="false">
  160. <label class="md-nav__title" for="__nav_3">
  161. <span class="md-nav__icon md-icon"></span>
  162. Installation
  163. </label>
  164. <ul class="md-nav__list" data-md-scrollfix>
  165. <li class="md-nav__item">
  166. <a href="../../setup/installation/" class="md-nav__link">
  167. <span class="md-ellipsis">
  168. Installation
  169. </span>
  170. </a>
  171. </li>
  172. <li class="md-nav__item">
  173. <a href="../../setup/upgrade/" class="md-nav__link">
  174. <span class="md-ellipsis">
  175. Upgrade
  176. </span>
  177. </a>
  178. </li>
  179. <li class="md-nav__item">
  180. <a href="../../setup/build/" class="md-nav__link">
  181. <span class="md-ellipsis">
  182. Build
  183. </span>
  184. </a>
  185. </li>
  186. </ul>
  187. </nav>
  188. </li>
  189. <li class="md-nav__item">
  190. <a href="../../terminology/" class="md-nav__link">
  191. <span class="md-ellipsis">
  192. Terminology
  193. </span>
  194. </a>
  195. </li>
  196. <li class="md-nav__item md-nav__item--nested">
  197. <input class="md-nav__toggle md-toggle " type="checkbox" id="__nav_5" >
  198. <label class="md-nav__link" for="__nav_5" id="__nav_5_label" tabindex="0">
  199. <span class="md-ellipsis">
  200. Config
  201. </span>
  202. <span class="md-nav__icon md-icon"></span>
  203. </label>
  204. <nav class="md-nav" data-md-level="1" aria-labelledby="__nav_5_label" aria-expanded="false">
  205. <label class="md-nav__title" for="__nav_5">
  206. <span class="md-nav__icon md-icon"></span>
  207. Config
  208. </label>
  209. <ul class="md-nav__list" data-md-scrollfix>
  210. <li class="md-nav__item">
  211. <a href="../../config/options/" class="md-nav__link">
  212. <span class="md-ellipsis">
  213. Options
  214. </span>
  215. </a>
  216. </li>
  217. <li class="md-nav__item">
  218. <a href="../../config/deprecated_options/" class="md-nav__link">
  219. <span class="md-ellipsis">
  220. Deprecated Options
  221. </span>
  222. </a>
  223. </li>
  224. <li class="md-nav__item">
  225. <a href="../../config/branches/" class="md-nav__link">
  226. <span class="md-ellipsis">
  227. branches
  228. </span>
  229. </a>
  230. </li>
  231. <li class="md-nav__item">
  232. <a href="../../config/functions_categories_and_policies/" class="md-nav__link">
  233. <span class="md-ellipsis">
  234. functions, categories and policies
  235. </span>
  236. </a>
  237. </li>
  238. <li class="md-nav__item">
  239. <a href="../../config/func_readdir/" class="md-nav__link">
  240. <span class="md-ellipsis">
  241. func.readdir
  242. </span>
  243. </a>
  244. </li>
  245. <li class="md-nav__item">
  246. <a href="../../config/rename_and_link/" class="md-nav__link">
  247. <span class="md-ellipsis">
  248. rename and link
  249. </span>
  250. </a>
  251. </li>
  252. <li class="md-nav__item">
  253. <a href="../../config/cache/" class="md-nav__link">
  254. <span class="md-ellipsis">
  255. caching
  256. </span>
  257. </a>
  258. </li>
  259. <li class="md-nav__item">
  260. <a href="../../config/readahead/" class="md-nav__link">
  261. <span class="md-ellipsis">
  262. readahead
  263. </span>
  264. </a>
  265. </li>
  266. <li class="md-nav__item">
  267. <a href="../../config/inodecalc/" class="md-nav__link">
  268. <span class="md-ellipsis">
  269. inodecalc
  270. </span>
  271. </a>
  272. </li>
  273. <li class="md-nav__item">
  274. <a href="../../config/threads/" class="md-nav__link">
  275. <span class="md-ellipsis">
  276. threads
  277. </span>
  278. </a>
  279. </li>
  280. <li class="md-nav__item">
  281. <a href="../../config/pin-threads/" class="md-nav__link">
  282. <span class="md-ellipsis">
  283. pin-threads
  284. </span>
  285. </a>
  286. </li>
  287. <li class="md-nav__item">
  288. <a href="../../config/link_cow/" class="md-nav__link">
  289. <span class="md-ellipsis">
  290. link_cow
  291. </span>
  292. </a>
  293. </li>
  294. <li class="md-nav__item">
  295. <a href="../../config/fuse_msg_size/" class="md-nav__link">
  296. <span class="md-ellipsis">
  297. fuse_msg_size
  298. </span>
  299. </a>
  300. </li>
  301. <li class="md-nav__item">
  302. <a href="../../config/follow-symlinks/" class="md-nav__link">
  303. <span class="md-ellipsis">
  304. follow-symlinks
  305. </span>
  306. </a>
  307. </li>
  308. <li class="md-nav__item">
  309. <a href="../../config/link-exdev/" class="md-nav__link">
  310. <span class="md-ellipsis">
  311. link-exdev
  312. </span>
  313. </a>
  314. </li>
  315. <li class="md-nav__item">
  316. <a href="../../config/rename-exdev/" class="md-nav__link">
  317. <span class="md-ellipsis">
  318. rename-exdev
  319. </span>
  320. </a>
  321. </li>
  322. <li class="md-nav__item">
  323. <a href="../../config/symlinkify/" class="md-nav__link">
  324. <span class="md-ellipsis">
  325. symlinkify
  326. </span>
  327. </a>
  328. </li>
  329. <li class="md-nav__item">
  330. <a href="../../config/nullrw/" class="md-nav__link">
  331. <span class="md-ellipsis">
  332. nullrw
  333. </span>
  334. </a>
  335. </li>
  336. <li class="md-nav__item">
  337. <a href="../../config/xattr/" class="md-nav__link">
  338. <span class="md-ellipsis">
  339. xattr
  340. </span>
  341. </a>
  342. </li>
  343. <li class="md-nav__item">
  344. <a href="../../config/nfsopenhack/" class="md-nav__link">
  345. <span class="md-ellipsis">
  346. nfsopenhack
  347. </span>
  348. </a>
  349. </li>
  350. <li class="md-nav__item">
  351. <a href="../../config/statfs/" class="md-nav__link">
  352. <span class="md-ellipsis">
  353. statfs / statvfs
  354. </span>
  355. </a>
  356. </li>
  357. <li class="md-nav__item">
  358. <a href="../../config/flush-on-close/" class="md-nav__link">
  359. <span class="md-ellipsis">
  360. flush-on-close
  361. </span>
  362. </a>
  363. </li>
  364. <li class="md-nav__item">
  365. <a href="../../config/export-support/" class="md-nav__link">
  366. <span class="md-ellipsis">
  367. export-support
  368. </span>
  369. </a>
  370. </li>
  371. </ul>
  372. </nav>
  373. </li>
  374. <li class="md-nav__item">
  375. <a href="../../error_handling_and_logging/" class="md-nav__link">
  376. <span class="md-ellipsis">
  377. Error Handling and Logging
  378. </span>
  379. </a>
  380. </li>
  381. <li class="md-nav__item">
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  515. Do hardlinks work?
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  520. <a href="#how-does-mergerfs-handle-moving-and-copying-of-files" class="md-nav__link">
  521. <span class="md-ellipsis">
  522. How does mergerfs handle moving and copying of files?
  523. </span>
  524. </a>
  525. </li>
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  527. <a href="#does-ficlone-or-ficlonerange-work" class="md-nav__link">
  528. <span class="md-ellipsis">
  529. Does FICLONE or FICLONERANGE work?
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  535. <span class="md-ellipsis">
  536. 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?
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  542. <span class="md-ellipsis">
  543. Why does the total available space in mergerfs not equal outside?
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  548. <a href="#i-notice-massive-slowdowns-of-writes-when-enabling-cachefiles" class="md-nav__link">
  549. <span class="md-ellipsis">
  550. I notice massive slowdowns of writes when enabling cache.files.
  551. </span>
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  562. <a href="#why-use-fuse-why-not-a-kernel-based-solution" class="md-nav__link">
  563. <span class="md-ellipsis">
  564. Why use FUSE? Why not a kernel based solution?
  565. </span>
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  569. <a href="#is-my-oss-libfuse-needed-for-mergerfs-to-work" class="md-nav__link">
  570. <span class="md-ellipsis">
  571. Is my OS's libfuse needed for mergerfs to work?
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  576. <a href="#why-was-splice-support-removed" class="md-nav__link">
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  644. <li class="md-nav__item">
  645. <a href="#do-hardlinks-work" class="md-nav__link">
  646. <span class="md-ellipsis">
  647. Do hardlinks work?
  648. </span>
  649. </a>
  650. </li>
  651. <li class="md-nav__item">
  652. <a href="#how-does-mergerfs-handle-moving-and-copying-of-files" class="md-nav__link">
  653. <span class="md-ellipsis">
  654. How does mergerfs handle moving and copying of files?
  655. </span>
  656. </a>
  657. </li>
  658. <li class="md-nav__item">
  659. <a href="#does-ficlone-or-ficlonerange-work" class="md-nav__link">
  660. <span class="md-ellipsis">
  661. Does FICLONE or FICLONERANGE work?
  662. </span>
  663. </a>
  664. </li>
  665. <li class="md-nav__item">
  666. <a href="#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" class="md-nav__link">
  667. <span class="md-ellipsis">
  668. 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?
  669. </span>
  670. </a>
  671. </li>
  672. <li class="md-nav__item">
  673. <a href="#why-does-the-total-available-space-in-mergerfs-not-equal-outside" class="md-nav__link">
  674. <span class="md-ellipsis">
  675. Why does the total available space in mergerfs not equal outside?
  676. </span>
  677. </a>
  678. </li>
  679. <li class="md-nav__item">
  680. <a href="#i-notice-massive-slowdowns-of-writes-when-enabling-cachefiles" class="md-nav__link">
  681. <span class="md-ellipsis">
  682. I notice massive slowdowns of writes when enabling cache.files.
  683. </span>
  684. </a>
  685. </li>
  686. <li class="md-nav__item">
  687. <a href="#why-cant-i-see-my-files-directories" class="md-nav__link">
  688. <span class="md-ellipsis">
  689. Why can't I see my files / directories?
  690. </span>
  691. </a>
  692. </li>
  693. <li class="md-nav__item">
  694. <a href="#why-use-fuse-why-not-a-kernel-based-solution" class="md-nav__link">
  695. <span class="md-ellipsis">
  696. Why use FUSE? Why not a kernel based solution?
  697. </span>
  698. </a>
  699. </li>
  700. <li class="md-nav__item">
  701. <a href="#is-my-oss-libfuse-needed-for-mergerfs-to-work" class="md-nav__link">
  702. <span class="md-ellipsis">
  703. Is my OS's libfuse needed for mergerfs to work?
  704. </span>
  705. </a>
  706. </li>
  707. <li class="md-nav__item">
  708. <a href="#why-was-splice-support-removed" class="md-nav__link">
  709. <span class="md-ellipsis">
  710. Why was splice support removed?
  711. </span>
  712. </a>
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  714. <li class="md-nav__item">
  715. <a href="#how-does-mergerfs-handle-credentials" class="md-nav__link">
  716. <span class="md-ellipsis">
  717. How does mergerfs handle credentials?
  718. </span>
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  733. </a>
  734. <h1 id="technical-behavior-and-limitations">Technical Behavior and Limitations</h1>
  735. <h2 id="do-hardlinks-work">Do hardlinks work?</h2>
  736. <p>Yes. See also the option <code>inodecalc</code> for how inode values are
  737. calculated.</p>
  738. <p>What mergerfs does not do is fake hard links across branches. Read
  739. the section "rename &amp; link" for how it works.</p>
  740. <p>Remember that hardlinks will NOT work across devices. That includes
  741. between the original filesystem and a mergerfs pool, between two
  742. separate pools of the same underlying filesystems, or bind mounts of
  743. paths within the mergerfs pool. The latter is common when using Docker
  744. or Podman. Multiple volumes (bind mounts) to the same underlying
  745. filesystem are considered different devices. There is no way to link
  746. between them. You should mount in the highest directory in the
  747. mergerfs pool that includes all the paths you need if you want links
  748. to work.</p>
  749. <h2 id="how-does-mergerfs-handle-moving-and-copying-of-files">How does mergerfs handle moving and copying of files?</h2>
  750. <p>This is a <em>very</em> common mistaken assumption regarding how filesystems
  751. work. There is no such thing as "move" or "copy." These concepts are
  752. high level behaviors made up of numerous independent steps and <em>not</em>
  753. individual filesystem functions.</p>
  754. <p>A "move" can include a "copy" so lets describe copy first.</p>
  755. <p>When an application copies a file from source to destination it can do
  756. so in a number of ways but the basics are the following.</p>
  757. <ol>
  758. <li><code>open</code> the source file.</li>
  759. <li><code>create</code> the destination file.</li>
  760. <li><code>read</code> a chunk of data from source and <code>write</code> to
  761. destination. Continue till it runs out of data to copy.</li>
  762. <li>Copy file metadata (<code>stat</code>) such as ownership (<code>chown</code>),
  763. permissions (<code>chmod</code>), timestamps (<code>utimes</code>), extended attributes
  764. (<code>getxattr</code>, <code>setxattr</code>), etc.</li>
  765. <li><code>close</code> source and destination files.</li>
  766. </ol>
  767. <p>"move" is typically a <code>rename(src,dst)</code> and if that errors with
  768. <code>EXDEV</code> (meaning the source and destination are on different
  769. filesystems) the application will "copy" the file as described above
  770. and then it removes (<code>unlink</code>) the source.</p>
  771. <p>The <code>rename(src,dst)</code>, <code>open(src)</code>, <code>create(dst)</code>, data copying,
  772. metadata copying, <code>unlink(src)</code>, etc. are entirely distinct and
  773. separate events. There is really no practical way to know that what is
  774. ultimately occurring is the "copying" of a file or what the source
  775. file would be. Since the source is not known there is no way to know
  776. how large a created file is destined to become. This is why it is
  777. impossible for mergerfs to choose the branch for a <code>create</code> based on
  778. file size. The only context provided when a file is created, besides
  779. the name, is the permissions, if it is to be read and/or written, and
  780. some low level settings for the operating system.</p>
  781. <p>All of this means that mergerfs can not make decisions when a file is
  782. created based on file size or the source of the data. That information
  783. is simply not available. At best mergerfs could respond to files
  784. reaching a certain size when writing data or when a file is closed.</p>
  785. <p>Related: if a user wished to have mergerfs perform certain activities
  786. based on the name of a file it is common and even best practice for a
  787. program to write to a temporary file first and then rename to its
  788. final destination. That temporary file name will typically be random
  789. and have no indication of the type of file being written.</p>
  790. <h2 id="does-ficlone-or-ficlonerange-work">Does FICLONE or FICLONERANGE work?</h2>
  791. <p>Unfortunately not. FUSE, the technology mergerfs is based on, does not
  792. support the <code>clone_file_range</code> feature needed for it to work. mergerfs
  793. won't even know such a request is made. The kernel will simply return
  794. an error back to the application making the request.</p>
  795. <p>Should FUSE gain the ability mergerfs will be updated to support it.</p>
  796. <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>
  797. <p>First make sure you've read the sections above about policies, path
  798. preservation, branch filtering, and the options <strong>minfreespace</strong>,
  799. <strong>moveonenospc</strong>, <strong>statfs</strong>, and <strong>statfs_ignore</strong>.</p>
  800. <p>mergerfs is simply presenting a union of the content within multiple
  801. branches. The reported free space is an aggregate of space available
  802. within the pool (behavior modified by <strong>statfs</strong> and
  803. <strong>statfs_ignore</strong>). It does not represent a contiguous space. In the
  804. same way that read-only filesystems, those with quotas, or reserved
  805. space report the full theoretical space available.</p>
  806. <p>Due to path preservation, branch tagging, read-only status, and
  807. <strong>minfreespace</strong> settings it is perfectly valid that <code>ENOSPC</code> / "out
  808. of space" / "no space left on device" be returned. It is doing what
  809. was asked of it: filtering possible branches due to those
  810. settings. Only one error can be returned and if one of the reasons for
  811. filtering a branch was <strong>minfreespace</strong> then it will be returned as
  812. such. <strong>moveonenospc</strong> is only relevant to writing a file which is too
  813. large for the filesystem it's currently on.</p>
  814. <p>It is also possible that the filesystem selected has run out of
  815. inodes. Use <code>df -i</code> to list the total and available inodes per
  816. filesystem.</p>
  817. <p>If you don't care about path preservation then simply change the
  818. <code>create</code> policy to one which isn't. <code>mfs</code> is probably what most are
  819. looking for. The reason it's not default is because it was originally
  820. set to <code>epmfs</code> and changing it now would change people's setup. Such a
  821. setting change will likely occur in mergerfs 3.</p>
  822. <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>
  823. <p>Are you using ext2/3/4? With reserve for root? mergerfs uses available
  824. space for statfs calculations. If you've reserved space for root then
  825. it won't show up.</p>
  826. <p>You can remove the reserve by running: <code>tune2fs -m 0 &lt;device&gt;</code></p>
  827. <h2 id="i-notice-massive-slowdowns-of-writes-when-enabling-cachefiles">I notice massive slowdowns of writes when enabling cache.files.</h2>
  828. <p>When file caching is enabled in any form (<code>cache.files!=off</code>) it will
  829. issue <code>getxattr</code> requests for <code>security.capability</code> prior to <em>every
  830. single write</em>. This will usually result in performance degradation,
  831. especially when using a network filesystem (such as NFS or SMB.)
  832. Unfortunately at this moment, the kernel is not caching the response.</p>
  833. <p>To work around this situation mergerfs offers a few solutions.</p>
  834. <ol>
  835. <li>Set <code>security_capability=false</code>. It will short circuit any call and
  836. return <code>ENOATTR</code>. This still means though that mergerfs will
  837. receive the request before every write but at least it doesn't get
  838. passed through to the underlying filesystem.</li>
  839. <li>Set <code>xattr=noattr</code>. Same as above but applies to <em>all</em> calls to
  840. getxattr. Not just <code>security.capability</code>. This will not be cached
  841. by the kernel either but mergerfs' runtime config system will still
  842. function.</li>
  843. <li>Set <code>xattr=nosys</code>. Results in mergerfs returning <code>ENOSYS</code> which
  844. <em>will</em> be cached by the kernel. No future xattr calls will be
  845. forwarded to mergerfs. The downside is that also means the xattr
  846. based config and query functionality won't work either.</li>
  847. <li>Disable file caching. If you aren't using applications which use
  848. <code>mmap</code> it's probably simpler to just disable it altogether. The
  849. kernel won't send the requests when caching is disabled.</li>
  850. </ol>
  851. <h2 id="why-cant-i-see-my-files-directories">Why can't I see my files / directories?</h2>
  852. <p>It's almost always a permissions issue. Unlike mhddfs and
  853. unionfs-fuse, which runs as root and attempts to access content as
  854. such, mergerfs always changes its credentials to that of the
  855. caller. This means that if the user does not have access to a file or
  856. directory than neither will mergerfs. However, because mergerfs is
  857. creating a union of paths it may be able to read some files and
  858. directories on one filesystem but not another resulting in an
  859. incomplete set.</p>
  860. <p>Whenever you run into a split permission issue (seeing some but not
  861. all files) try using
  862. <a href="https://github.com/trapexit/mergerfs-tools">mergerfs.fsck</a> tool to
  863. check for and fix the mismatch. If you aren't seeing anything at all
  864. be sure that the basic permissions are correct. The user and group
  865. values are correct and that directories have their executable bit
  866. set. A common mistake by users new to Linux is to <code>chmod -R 644</code> when
  867. they should have <code>chmod -R u=rwX,go=rX</code>.</p>
  868. <p>If using a network filesystem such as NFS or SMB (Samba) be sure to
  869. pay close attention to anything regarding permissioning and
  870. users. Root squashing and user translation for instance has bitten a
  871. few mergerfs users. Some of these also affect the use of mergerfs from
  872. container platforms such as Docker.</p>
  873. <h2 id="why-use-fuse-why-not-a-kernel-based-solution">Why use FUSE? Why not a kernel based solution?</h2>
  874. <p>As with any solution to a problem, there are advantages and
  875. disadvantages to each one.</p>
  876. <p>A FUSE based solution has all the downsides of FUSE:</p>
  877. <ul>
  878. <li>Higher IO latency due to the trips in and out of kernel space</li>
  879. <li>Higher general overhead due to trips in and out of kernel space</li>
  880. <li>Double caching when using page caching</li>
  881. <li>Misc limitations due to FUSE's design</li>
  882. </ul>
  883. <p>But FUSE also has a lot of upsides:</p>
  884. <ul>
  885. <li>Easier to offer a cross platform solution</li>
  886. <li>Easier forward and backward compatibility</li>
  887. <li>Easier updates for users</li>
  888. <li>Easier and faster release cadence</li>
  889. <li>Allows more flexibility in design and features</li>
  890. <li>Overall easier to write, secure, and maintain</li>
  891. <li>Much lower barrier to entry (getting code into the kernel takes a
  892. lot of time and effort initially)</li>
  893. </ul>
  894. <h2 id="is-my-oss-libfuse-needed-for-mergerfs-to-work">Is my OS's libfuse needed for mergerfs to work?</h2>
  895. <p>No. Normally <code>mount.fuse</code> is needed to get mergerfs (or any FUSE
  896. filesystem to mount using the <code>mount</code> command but in vendoring the
  897. libfuse library the <code>mount.fuse</code> app has been renamed to
  898. <code>mount.mergerfs</code> meaning the filesystem type in <code>fstab</code> can simply be
  899. <code>mergerfs</code>. That said there should be no harm in having it installed
  900. and continuing to using <code>fuse.mergerfs</code> as the type in <code>/etc/fstab</code>.</p>
  901. <p>If <code>mergerfs</code> doesn't work as a type it could be due to how the
  902. <code>mount.mergerfs</code> tool was installed. Must be in <code>/sbin/</code> with proper
  903. permissions.</p>
  904. <h2 id="why-was-splice-support-removed">Why was splice support removed?</h2>
  905. <p>After a lot of testing over the years, splicing always appeared to
  906. at best, provide equivalent performance, and in some cases, worse
  907. performance. Splice is not supported on other platforms forcing a
  908. traditional read/write fallback to be provided. The splice code was
  909. removed to simplify the codebase.</p>
  910. <h2 id="how-does-mergerfs-handle-credentials">How does mergerfs handle credentials?</h2>
  911. <p>mergerfs is a multithreaded application in order to handle requests
  912. from the kernel concurrently. Each FUSE message has a header with
  913. certain details about the request include the process ID (pid) of the
  914. requestion application, the process' effective user id (uid), and
  915. group id (gid). To ensure proper POSIX filesystem behavior and
  916. security mergerfs must change its identity to match that of the
  917. requester when performing the core filesystem function on the
  918. underlying filesystem. On most Unix/POSIX based system a process and
  919. all its threads are under the same uid and gid. However, on Linux each
  920. thread may have its own credentials. This allows mergerfs to be
  921. multithreaded and for each thread to change to the credentials
  922. (seteuid,setegid) as required by the incoming message it is
  923. handling. However, on FreeBSD this is not possible at the moment
  924. (though there has been
  925. <a href="https://wiki.freebsd.org/Per-Thread%20Credentials">discussions</a> and
  926. as such must change the credentials of the whole application when
  927. actioning messages. mergerfs does optimize this behavior by only
  928. changing credentials and locking the thread to do so if the process is
  929. currently not the same as what is necessary by the incoming request.</p>
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