Data Recovery from Linux filesystem corruption

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The structure of the Linux file system

The Linux file system treats everything as a file. This includes images, text files, programs, directories, partitions and hardware device drivers.

Each filesystem contains a control block, which holds information about that filesystem. The other blocks in the filesystem are inodes, which contain information about individual files, and data blocks, which contain the information stored in the individual files.

There is a substantial difference between the way the user sees the Linux filesystem (first sense) and the way the kernel (the core of a Linux system) actually stores the files. To the user, the filesystem appears as a hierarchical arrangement of directories that contain files and other directories (i.e., subdirectories). Directories and files are identified by their names. This hierarchy starts from a single directory called root, which is represented by a "/" (forward slash).

(The meaning of root and "/" are often confusing to new users of Linux. This because each has two distinct usages. The other meaning of root is a user who has administrative privileges on the computer, in contrast to ordinary users, who have only limited privileges in order to protect system security. The other use of "/" is as a separator between directories or between a directory and a file, similar to the backward slash used in MS-DOS.)

The Filesystem Hierarchy Standard (FHS) defines the main directories and their contents in Linux and other Unix-like operating systems. All files and directories appear under the root directory, even if they are stored on different physical devices (e.g., on different disks or on different computers). A few of the directories defined by the FHS are /bin (command binaries for all users), /boot (boot loader files such as the kernel), /home (users home directories), /mnt (for mounting a CDROM or floppy disk), /root (home directory for the root user), /sbin (executables used only by the root user) and /usr (where most application programs get installed).

To the Linux kernel, however, the filesystem is flat. That is, it does not:

• have a hierarchical structure
• differentiate between directories, files or programs
• identify files by names. Instead, the kernel uses inodes to represent each file.

• its inode number (a unique identification number)
• the owner and group associated with the file
• the file type (for example, whether it is a regular file or a directory)
• the file's permission list
• the file creation, access and modification times
• the size of the file
• the disk address (i.e., the location on the disk where the file is physically stored).