02struct.txt

# Copyright (C) 2005-2019 Junjiro R. Okajima

Basic Aufs Internal Structure

Superblock/Inode/Dentry/File Objects
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As like an ordinary filesystem, aufs has its own
superblock/inode/dentry/file objects. All these objects have a
dynamically allocated array and store the same kind of pointers to the
lower filesystem, branch.
For example, when you build a union with one readwrite branch and one
readonly, mounted /au, /rw and /ro respectively.
- /au = /rw + /ro
- /ro/fileA exists but /rw/fileA

Aufs lookup operation finds /ro/fileA and gets dentry for that. These
pointers are stored in a aufs dentry. The array in aufs dentry will be,
- [0] = NULL (because /rw/fileA doesn't exist)
- [1] = /ro/fileA

This style of an array is essentially same to the aufs
superblock/inode/dentry/file objects.

Because aufs supports manipulating branches, ie. add/delete/change
branches dynamically, these objects has its own generation. When
branches are changed, the generation in aufs superblock is
incremented. And a generation in other object are compared when it is
accessed. When a generation in other objects are obsoleted, aufs
refreshes the internal array.


Superblock
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Additionally aufs superblock has some data for policies to select one
among multiple writable branches, XIB files, pseudo-links and kobject.
See below in detail.
About the policies which supports copy-down a directory, see
wbr_policy.txt too.