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linux文件系統(tǒng)知識(shí)

2013.05.02

The superblock is essentially file system metadata and defines the file system type, size, status, and information about other metadata structures (metadata of metadata). The superblock is very critical to the file system and therefore is stored in multiple redundant copies for each file system. The superblock is a very "high level" metadata structure for the file system. For example, if the superblock of a partition, /var, becomes corrupt then the file system in question (/var) cannot be mounted by the operating system. Commonly in this event fsck is run and will automatically select an alternate, backup copy of the superblock and attempt to recover the file system. The backup copies themselves are stored in block groups spread through the file system with the first stored at a 1 block offset from the start of the partition. This is important in the event that a manual recovery is necessary. You may view information about superblock backups with the command dumpe2fs /dev/foo | grep -i superblock which is useful in the event of a manual recovery attempt. Let us suppose that the dumpe2fs command outputs the line Backup superblock at 163840, Group descriptors at 163841-163841. We can use this information, and additional knowledge about the file system structure, to attempt to use this superblock backup: /sbin/fsck.ext3 -b 163840 -B 1024 /dev/foo. Please note that I have assumed a block size of 1024 bytes for this example.

Superblock 是文件系統(tǒng)最基本的元數(shù)據(jù)，它定義了文件系統(tǒng)的類似、大小、狀態(tài)，和其他元數(shù)據(jù)結(jié)構(gòu)的信息（元數(shù)據(jù)的元數(shù)據(jù)）。Superblock 對(duì)于文件系統(tǒng)來說是非常關(guān)鍵的，因此對(duì)于每個(gè)文件系統(tǒng)它都冗余存儲(chǔ)了多份。Superblock對(duì)于文件系統(tǒng)來說是一個(gè)非?！案叩燃?jí)”的元數(shù)據(jù)結(jié)構(gòu)。例如，如果 /var 分區(qū)的 Superblock 損壞了，那么 /var 分區(qū)將無法掛載。在這時(shí)候，一般會(huì)執(zhí)行 fsck 來自動(dòng)選擇一份 Superblock 備份來替換損壞的 Superblock，并嘗試修復(fù)文件系統(tǒng)。主 Superblock 存儲(chǔ)在分區(qū)的 block 0 或者 block 1 中，而 Superblock 的備份則分散存儲(chǔ)在文件系統(tǒng)的多組 block 中。當(dāng)需要手工恢復(fù)時(shí)，我們可以使用 dumpe2fs /dev/sda1 | grep -i superblock 來查看 sda1 分區(qū)的 superblock 備份有哪一份是可用的。我們假設(shè) dumpe2fs 輸出了這樣一行：Backup superblock at 163840, Group descriptors at 163841-163841 ，通過這條信息，我們就可以嘗試使用這個(gè) superblock 備份：/sbin/fsck.ext3 -b 163840 -B 1024/dev/sda1。請(qǐng)注意，這里我們假設(shè) block 的大小為 1024 字節(jié)。

對(duì)于特定的文件系統(tǒng)，此文件系統(tǒng)的所有super block都保存在file_system_type中的fs_supers鏈表中。所有的文件系統(tǒng)都保存在file_systems鏈表中。

An inode exists in, or on, a file system and represents metadata about a file. For clarity, all objects in a Linux or UNIX system are files; actual files, directories, devices, and so on. Please note that, among the metadata contained in an inode, there is no file name as humans think of it, this will be important later. An inode contains essentially information about ownership (user, group), access mode (read, write, execute permissions) and file type

Inode 中包含了一個(gè)文件的元數(shù)據(jù)，在Linux/Unix 系統(tǒng)中的所有對(duì)象均為文件，在 Inode 所包含的元數(shù)據(jù)中，并沒有文件名。一個(gè) Inode 包含的基本信息有：所有權(quán)（用戶，組），訪問模式（讀、寫、執(zhí)行權(quán)限）和文件類型。

A dentry is the glue that holds inodes and files together by relating inode numbers to file names. Dentries also play a role in directory caching which, ideally, keeps the most frequently used files on-hand for faster access. File system traversal is another aspect of the dentry as it maintains a relationship between directories and their files

Dentry 將 Inode number 和文件名聯(lián)系起來。這個(gè)索引節(jié)點(diǎn)可以是文件，也可以是目錄。 inode（可理解為ext2 inode）對(duì)應(yīng)于物理磁盤上的具體對(duì)象，dentry是一個(gè)內(nèi)存實(shí)體，其中的d_inode成員指向?qū)?yīng)的inode。也就是說，一個(gè)inode可以在運(yùn)行的時(shí)候鏈接多個(gè)dentry，而d_count記錄了這個(gè)鏈接的數(shù)量。

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