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Comment by Tobu | Hacker News Reader

I'm not sure where the confusion is coming from.

If userspace wants to distinguish files, it normally does so with a (ino_t, dev_t) pair. With some nuance wrt inode generations if you aren't holding on to files yet want to guard against ino reuse, and some funkyness with overlayfs.

Most filesystems don't expose a way to access files by ino only, so the internal implementation generally doesn't key anything by them or rely on them itself; they're just a kind of of uniqueness cookie. But keying by them somewhere in the kernel implementation is feasible, if you scope it properly.

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It may be that, in modern times, filesystems don't actually key things with i-node number. But wasn't that the original purpose? Otherwise, why the design where directories are basically a mapping of names to i-node numbers?

Also, the article does say this:

> On non-volatile filesystems, inode numbers are typically finite and often have some kind of implementation-defined semantic meaning (eg. implying where the inode is disk).

If you've got other means to know where the i-node is on disk, then why would a filesystem bother encoding that into the i-node numbering scheme?

Maybe the article is wrong here. I don't know. But if the question is where the confusion is coming from, the article definitely seems to be painting a picture where normal filesystems look up things based on i-node number and tmpfs is relatively unique in not doing so.

Yes, the OG Unix file system simply had a fixed-size (at partition time, this is still the case for ext4 and XFS iirc) array of struct inode on the disk, and the inode number (iirc it was just idx or something like that in the code) is simply the index into that array. An OG Unix directory was simply a file with the directory flag set and the file was just an array of struct { char name[30]; int ino; } (~something like that). You actually used to be able to just open() a directory and directly read directory entries (dirents henceforth) from it, just like a file - exactly like a file, because it WAS a file.

ext and XFS still largely work like this, though directories are now hashtables and I think XFS supports multiple independent arrays for inodes (or just for extent allocation? I don't remember). NTFS also looks a lot like a Unix file system with some weird growths on it, and not a whole lot like FAT.

There's also some funkyness with btrfs subvolumes:

https://news.ycombinator.com/item?id=28274054

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