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I'm learning about Linux filesystems, with these sources:
http://linuxgazette.net/issue21/ext2.html
http://homepage.smc.edu/morgan_david/cs40/analyze-ext2.htm
But I have one question about the root directory: why is its inode number always two? Why not one, or another number?
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The filesystem root directory is inode 2. The meaning of particular inode numbers differs by filesystem.
Inode 1 is used to keep track of any bad blocks on the disk; it is essentially a hidden file containing the bad blocks. Those bad blocks which are recorded using e2fsck -c . Inode 2 is used by the root directory, and indicates starting of filesystem inodes.
2 files can have the same inode, but only if they are part of different partitions. Inodes are only unique on a partition level, not on the whole system.
To get the number of inodes of files in a directory, for example, the root directory, open a terminal window and run the following ls command, where the -l option means long listing format, -a means all files and -i mean to print the index number of each file.
The first inode number is 1. 0 is used as a NULL value, to indicate that there is no inode. Inode 1 is used to keep track of any bad blocks on the disk; it is essentially a hidden file containing the bad blocks, so that they will not be used by another file. The bad blocks can be recorded using e2fsck -c. The filesystem root directory is inode 2.
The meaning of particular inode numbers differs by filesystem. For ext4 you can find more information on the Ext4 Wiki Ext4 Disk Layout page; in particular see the "Special inodes" table.
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