linux文件系统知识

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 是文件系统最基本的元数据，它定义了文件系统的类似、大小、状态，和其他元数据结构的信息（元数据的元数据）。Superblock 对于文件系统来说是非常关键的，因此对于每个文件系统它都冗余存储了多份。Superblock对于文件系统来说是一个非常“高等级”的元数据结构。例如，如果 /var 分区的 Superblock 损坏了，那么 /var 分区将无法挂载。在这时候，一般会执行 fsck 来自动选择一份 Superblock 备份来替换损坏的 Superblock，并尝试修复文件系统。主 Superblock 存储在分区的 block 0 或者 block 1 中，而 Superblock 的备份则分散存储在文件系统的多组 block 中。当需要手工恢复时，我们可以使用 dumpe2fs /dev/sda1 | grep -i superblock 来查看 sda1 分区的 superblock 备份有哪一份是可用的。我们假设 dumpe2fs 输出了这样一行：Backup superblock at 163840, Group descriptors at 163841-163841 ，通过这条信息，我们就可以尝试使用这个 superblock 备份：/sbin/fsck.ext3 -b 163840 -B 1024/dev/sda1。请注意，这里我们假设 block 的大小为 1024 字节。

对于特定的文件系统，此文件系统的所有super block都保存在file_system_type中的fs_supers链表中。所有的文件系统都保存在file_systems链表中。

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 中包含了一个文件的元数据，在Linux/Unix 系统中的所有对象均为文件，在 Inode 所包含的元数据中，并没有文件名。一个 Inode 包含的基本信息有：所有权（用户，组），访问模式（读、写、执行权限）和文件类型。

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 将 Inode number 和文件名联系起来。这个索引节点可以是文件，也可以是目录。 inode（可理解为ext2 inode）对应于物理磁盘上的具体对象，dentry是一个内存实体，其中的d_inode成员指向对应的inode。也就是说，一个inode可以在 运行的时候链接多个dentry，而d_count记录了这个链接的数量。