一口氣搞懂「文件系統」，就靠這 20 張圖了

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BB，直接上「硬菜」。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/47/477dd4a8ce8bbadf1bdaf3a5e1f11776.png","alt":null,"title":"","style":[{"key":"width","value":"100%"},{"key":"bordertype","value":"none"}],"href":"","fromPaste":false,"pastePass":false}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"horizontalrule"},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"正文"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"文件系統的基本組成"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"文件系統是操作系統中負責管理持久數據的子系統，說簡單點，就是負責把用戶的文件存到磁盤硬件中，因爲即使計算機斷電了，磁盤裏的數據並不會丟失，所以可以持久化的保存文件。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"文件系統的基本數據單位是文件，它的目的是對磁盤上的文件進行組織管理，那組織的方式不同，就會形成不同的文件系統。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"Linux 最經典的一句話是：「"},{"type":"text","marks":[{"type":"strong"}],"text":"一切皆文件"},{"type":"text","text":"」，不僅普通的文件和目錄，就連塊設備、管道、socket 等，也都是統一交給文件系統管理的。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"Linux 文件系統會爲每個文件分配兩個數據結構：*"},{"type":"text","marks":[{"type":"italic"}],"text":"索引節點（"},{"type":"text","text":"index node"},{"type":"text","marks":[{"type":"italic"}],"text":"）和目錄項（"},{"type":"text","text":"directory entry"},{"type":"text","marks":[{"type":"italic"}],"text":"）"},{"type":"text","text":"*，它們主要用來記錄文件的元信息和目錄層次結構。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"索引節點，也就是 "},{"type":"text","marks":[{"type":"italic"}],"text":"inode"},{"type":"text","text":"，用來記錄文件的元信息，比如 inode 編號、文件大小、訪問權限、創建時間、修改時間、"},{"type":"text","marks":[{"type":"strong"}],"text":"數據在磁盤的位置"},{"type":"text","text":"等等。索引節點是文件的"},{"type":"text","marks":[{"type":"strong"}],"text":"唯一"},{"type":"text","text":"標識，它們之間一一對應，也同樣都會被存儲在硬盤中，所以"},{"type":"text","marks":[{"type":"strong"}],"text":"索引節點同樣佔用磁盤空間"},{"type":"text","text":"。"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"目錄項，也就是 "},{"type":"text","marks":[{"type":"italic"}],"text":"dentry"},{"type":"text","text":"，用來記錄文件的名字、"},{"type":"text","marks":[{"type":"strong"}],"text":"索引節點指針"},{"type":"text","text":"以及與其他目錄項的層級關聯關係。多個目錄項關聯起來，就會形成目錄結構，但它與索引節點不同的是，"},{"type":"text","marks":[{"type":"strong"}],"text":"目錄項是由內核維護的一個數據結構，不存放於磁盤，而是緩存在內存"},{"type":"text","text":"。"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"由於索引節點唯一標識一個文件，而目錄項記錄着文件的名，所以目錄項和索引節點的關係是多對一，也就是說，一個文件可以有多個別字。比如，硬鏈接的實現就是多個目錄項中的索引節點指向同一個文件。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"注意，目錄也是文件，也是用索引節點唯一標識，和普通文件不同的是，普通文件在磁盤裏面保存的是文件數據，而目錄文件在磁盤裏面保存子目錄或文件。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"blockquote","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"目錄項和目錄是一個東西嗎？"}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"雖然名字很相近，但是它們不是一個東西，目錄是個文件，持久化存儲在磁盤，而目錄項是內核一個數據結構，緩存在內存。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"如果查詢目錄頻繁從磁盤讀，效率會很低，所以內核會把已經讀過的目錄用目錄項這個數據結構緩存在內存，下次再次讀到相同的目錄時，只需從內存讀就可以，大大提高了文件系統的效率。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"注意，目錄項這個數據結構不只是表示目錄，也是可以表示文件的。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"blockquote","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"那文件數據是如何存儲在磁盤的呢？"}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"磁盤讀寫的最小單位是"},{"type":"text","marks":[{"type":"strong"}],"text":"扇區"},{"type":"text","text":"，扇區的大小隻有 "},{"type":"codeinline","content":[{"type":"text","text":"512B"}]},{"type":"text","text":" 大小，很明顯，如果每次讀寫都以這麼小爲單位，那這讀寫的效率會非常低。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"所以，文件系統把多個扇區組成了一個"},{"type":"text","marks":[{"type":"strong"}],"text":"邏輯塊"},{"type":"text","text":"，每次讀寫的最小單位就是邏輯塊（數據塊），Linux 中的邏輯塊大小爲 "},{"type":"codeinline","content":[{"type":"text","text":"4KB"}]},{"type":"text","text":"，也就是一次性讀寫 8 個扇區，這將大大提高了磁盤的讀寫的效率。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"以上就是索引節點、目錄項以及文件數據的關係，下面這個圖就很好的展示了它們之間的關係："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/0c/0c7d1e73a5eff79dbf126d10ba75e44b.png","alt":null,"title":"","style":[{"key":"width","value":"100%"},{"key":"bordertype","value":"none"}],"href":"","fromPaste":false,"pastePass":false}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"索引節點是存儲在硬盤上的數據，那麼爲了加速文件的訪問，通常會把索引節點加載到內存中。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"另外，磁盤進行格式化的時候，會被分成三個存儲區域，分別是超級塊、索引節點區和數據塊區。"}]},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"italic"}],"text":"超級塊"},{"type":"text","text":"，用來存儲文件系統的詳細信息，比如塊個數、塊大小、空閒塊等等。"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"italic"}],"text":"索引節點區"},{"type":"text","text":"，用來存儲索引節點；"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"italic"}],"text":"數據塊區"},{"type":"text","text":"，用來存儲文件或目錄數據；"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"我們不可能把超級塊和索引節點區全部加載到內存，這樣內存肯定撐不住，所以只有當需要使用的時候，纔將其加載進內存，它們加載進內存的時機是不同的："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"超級塊：當文件系統掛載時進入內存；"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"索引節點區：當文件被訪問時進入內存；"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"horizontalrule"},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"虛擬文件系統"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"文件系統的種類衆多，而操作系統希望"},{"type":"text","marks":[{"type":"strong"}],"text":"對用戶提供一個統一的接口"},{"type":"text","text":"，於是在用戶層與文件系統層引入了中間層，這個中間層就稱爲*"},{"type":"text","marks":[{"type":"italic"}],"text":"虛擬文件系統（"},{"type":"text","text":"Virtual File System，VFS"},{"type":"text","marks":[{"type":"italic"}],"text":"）。"},{"type":"text","text":"*"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"VFS 定義了一組所有文件系統都支持的數據結構和標準接口，這樣程序員不需要了解文件系統的工作原理，只需要瞭解 VFS 提供的統一接口即可。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"在 Linux 文件系統中，用戶空間、系統調用、虛擬機文件系統、緩存、文件系統以及存儲之間的關係如下圖："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/b9/b92499b17ed11bc279689ecdb6efb4b6.png","alt":null,"title":null,"style":null,"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"Linux 支持的文件系統也不少，根據存儲位置的不同，可以把文件系統分爲三類："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"italic"}],"text":"磁盤的文件系統"},{"type":"text","text":"，它是直接把數據存儲在磁盤中，比如 Ext 2/3/4、XFS 等都是這類文件系統。"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"italic"}],"text":"內存的文件系統"},{"type":"text","text":"，這類文件系統的數據不是存儲在硬盤的，而是佔用內存空間，我們經常用到的 "},{"type":"codeinline","content":[{"type":"text","text":"/proc"}]},{"type":"text","text":" 和 "},{"type":"codeinline","content":[{"type":"text","text":"/sys"}]},{"type":"text","text":" 文件系統都屬於這一類，讀寫這類文件，實際上是讀寫內核中相關的數據數據。"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"italic"}],"text":"網絡的文件系統"},{"type":"text","text":"，用來訪問其他計算機主機數據的文件系統，比如 NFS、SMB 等等。"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"文件系統首先要先掛載到某個目錄纔可以正常使用，比如 Linux 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open(name, flag); # 打開文件\n...\nwrite(fd,...); # 寫數據\n...\nclose(fd); # 關閉文件"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"上面簡單的代碼是讀取一個文件的過程："}]},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"首先用 "},{"type":"codeinline","content":[{"type":"text","text":"open"}]},{"type":"text","text":" 系統調用打開文件，"},{"type":"codeinline","content":[{"type":"text","text":"open"}]},{"type":"text","text":" 的參數中包含文件的路徑名和文件名。"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"使用 "},{"type":"codeinline","content":[{"type":"text","text":"write"}]},{"type":"text","text":" 寫數據，其中 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