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Introduction
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aufs [ei ju: ef es] | [a u f s]
1. abbrev. for "advanced multi-layered unification filesystem".
2. abbrev. for "another unionfs".
3. abbrev. for "auf das" in German which means "on the" in English.
   Ex. "Butter aufs Brot"(G) means "butter onto bread"(E).
   But "Filesystem aufs Filesystem" is hard to understand.

AUFS is a filesystem with features:
- multi layered stackable unification filesystem, the member directory
  is called as a branch.
- branch permission and attribute, 'readonly', 'real-readonly',
  'readwrite', 'whiteout-able', 'link-able whiteout' and their
  combination.
- internal "file copy-on-write".
- logical deletion, whiteout.
- dynamic branch manipulation, adding, deleting and changing permission.
- allow bypassing aufs, user's direct branch access.
- external inode number translation table and bitmap which maintains the
  persistent aufs inode number.
- seekable directory, including NFS readdir.
- file mapping, mmap and sharing pages.
- pseudo-link, hardlink over branches.
- loopback mounted filesystem as a branch.
- several policies to select one among multiple writable branches.
- revert a single systemcall when an error occurs in aufs.
- and more...


Multi Layered Stackable Unification Filesystem
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Most people already knows what it is.
It is a filesystem which unifies several directories and provides a
merged single directory. When users access a file, the access will be
passed/re-directed/converted (sorry, I am not sure which English word is
correct) to the real file on the member filesystem. The member
filesystem is called 'lower filesystem' or 'branch' and has a mode
'readonly' and 'readwrite.' And the deletion for a file on the lower
readonly branch is handled by creating 'whiteout' on the upper writable
branch.

On LKML, there have been discussions about UnionMount (Jan Blunck and
Bharata B Rao) and Unionfs (Erez Zadok). They took different approaches
to implement the merged-view.
The former tries putting it into VFS, and the latter implements as a
separate filesystem.
(If I misunderstand about these implementations, please let me know and
I shall correct it. Because it is a long time ago when I read their
source files last time).
UnionMount's approach will be able to small, but may be hard to share
branches between several UnionMount since the whiteout in it is
implemented in the inode on branch filesystem and always
shared. According to Bharata's post, readdir does not seems to be
finished yet.
Unionfs has a longer history. When I started implementing a stacking filesystem
(Aug 2005), it already existed. It has virtual super_block, inode,
dentry and file objects and they have an array pointing lower same kind
objects. After contributing many patches for Unionfs, I re-started my
project AUFS (Jun 2006).

In AUFS, the structure of filesystem resembles to Unionfs, but I
implemented my own ideas, approaches and enhancements and it became
totally different one.


Several characters/aspects of aufs
----------------------------------------------------------------------

Aufs has several characters or aspects.
1. a filesystem, callee of VFS helper
2. sub-VFS, caller of VFS helper for branches
3. a virtual filesystem which maintains persistent inode number
4. reader/writer of files on branches such like an application

1. Callee of VFS Helper
As an ordinary linux filesystem, aufs is a callee of VFS. For instance,
unlink(2) from an application reaches sys_unlink() kernel function and
then vfs_unlink() is called. vfs_unlink() is one of VFS helper and it
calls filesystem specific unlink operation. Actually aufs implements the
unlink operation but it behaves like a redirector.

2. Caller of VFS Helper for Branches
aufs_unlink() passes the unlink request to the branch filesystem as if
it were called from VFS. So the called unlink operation of the branch
filesystem acts as usual. As a caller of VFS helper, aufs should handle
every necessary pre/post operation for the branch filesystem.
- acquire the lock for the parent dir on a branch
- lookup in a branch
- revalidate dentry on a branch
- mnt_want_write() for a branch
- vfs_unlink() for a branch
- mnt_drop_write() for a branch
- release the lock on a branch

3. Persistent Inode Number
One of the most important issue for a filesystem is to maintain inode
numbers. This is particularly important to support exporting a
filesystem via NFS. Aufs is a virtual filesystem which doesn't have a
backend block device for its own. But some storage is necessary to
maintain inode number. It may be a large space and may not suit to keep
in memory. Aufs rents some space from its first writable branch
filesystem (by default) and creates file(s) on it. These files are
created by aufs internally and removed soon (currently) keeping opened.
Note: Because these files are removed, they are totally gone after
      unmounting aufs. It means the inode numbers are not persistent
      across unmount or reboot. I have a plan to make them really
      persistent which will be important for aufs on NFS server.

4. Read/Write Files Internally (copy-on-write)
Because a branch can be readonly, when you write a file on it, aufs will
"copy-up" it to the upper writable branch internally. And then write the
originally requested thing to the file. Generally kernel doesn't
open/read/write file actively. In aufs, even a single write may cause a
internal "file copy". This behaviour is very similar to cp(1) command.

Some people may think it is better to pass such work to user space
helper, instead of doing in kernel space. Actually I am still thinking
about it. But currently I have implemented it in kernel space.
