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Does a directory inode use a plain array storing ( filename, inode ) pairs, or some kind of associative array { filename: inode }to speed up file lookup, or any other optimizations on a plain array?

Take ext2/3/4 as example if you think the question is too broad or filesystem-specific.

This question is specifically focused on the internal representation of directory inodes.

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As we know, everything is a file in Linux even the directories. Also every file has an inode.

That said, a directory's inode has a map of it's contents (files and subdirectories) into blocks (a block is part of a inode structure), it can be described with the following image:

enter image description here

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    First of all, a directory’s inode does no such thing.  A directory’s inode points to the directory’s data blocks, which contain the inode numbers and filenames of the files that are “in” the directory.  Secondly, the OP clearly knows that a directory associates inode numbers with filenames — the question says so.  (The OP probably already knows everything you said.)  But your image illustrates the association at only a conceptual level.  The question is: how do the ext2/3/4 filesystems do this?  Your illustration might have been relatively accurate twenty years ago; it’s not very useful now. – G-Man Nov 23 '16 at 3:22

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