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On many *nix systems like OS X and Ubuntu, We can see the inode of root directory is 2. Then what is the inode 1 used for?

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    This will differ by filesystem; are you interested in a particular one? Commonly inode 1 is used for the list of bad blocks, but it's not required. – Michael Homer Apr 26 '15 at 8:49
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    This SO answer may be relevant: stackoverflow.com/a/2109363/2808351 – dhag Apr 27 '15 at 18:29
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Inode 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. Those bad blocks which are recorded using e2fsck -c.

Inode 2 is used by the root directory, and indicates starting of filesystem inodes.

  • We should note that except for 0 the values come from tradition and are not really fixed. For example minixfs has / as inode 1 and badblocks as whatever inode /.badblocks is. – Joshua Apr 6 '18 at 16:04
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In ext4 the Inode 1 is used for bad blocks. The link below the the kernel site describes which Inode is used for what purpose.

https://ext4.wiki.kernel.org/index.php/Ext4_Disk_Layout#Special_inodes

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