Linux File System

FILE SYSTEM

File system divided into two categories:

§  User data - stores actual data contained in files

§  Metadata - stores file system structural information such as superblock, inodes, directories

Understanding UNIX / Linux file system Superblock

Let us take an example of 20 GB hard disk. The entire disk space subdivided into multiple file system blocks. And blocks used for what?

Unix / Linux filesystem blocks

The blocks used for two different purpose:

1.    Most blocks stores user data aka files (user data).

2.    Some blocks in every file system store the file system's metadata. So what the hell is a metadata?

In simple words Metadata describes the structure of the file system. Most common metadata structure are superblock, inode and directories. Following paragraphs describes each of them.

Superblock

Each file system is different and they have type like ext2, ext3 etc. Further each file system has size like 5 GB, 10 GB and status such as mount status. In short each file system has a superblock, which contains information about file system such as:

§  File system type

§  Size

§  Status

§  Information about other metadata structures

Inodes

 

The inode (index node) is a fundamental concept in the Linux and UNIX filesystem. 

 Each and every file under Linux (and UNIX) has following attributes:

=> File type (executable, block special etc)
=> Permissions (read, write etc)
=> Owner
=> Group
=> File Size
=> File access, change and modification time (remember UNIX or Linux never stores file creation time, this is favorite question asked in UNIX/Linux sys admin job interview)
=> File deletion time

 

All the above information stored in an inode. In short the inode identifies the file and its attributes (as above) . Each inode is identified by a unique inode number within the file system. Inode is also know as index number.

How do I see file inode number?

You can use ls -i command to see inode number of file
$ ls -i /etc/passwd
Sample Output

32820 /etc/passwd

You can also use stat command to find out inode number and its attribute:
$ stat /etc/passwdOutput:

File: `/etc/passwd'

Size: 1988            Blocks: 8          IO Block: 4096   regular file

Device: 341h/833d       Inode: 32820       Links: 1

Access: (0644/-rw-r--r--)  Uid: (    0/    root)   Gid: (    0/    root)

Access: 2005-11-10 01:26:01.000000000 +0530

Modify: 2005-10-27 13:26:56.000000000 +0530

Change: 2005-10-27 13:26:56.000000000 +0530