
Aufs2 -- advanced multi layered unification filesystem version 2
http://aufs.sf.net
Junjiro R. Okajima


0. Introduction
----------------------------------------
In the early days, aufs was entirely re-designed and re-implemented
Unionfs Version 1.x series. After many original ideas, approaches,
improvements and implementations, it becomes totally different from
Unionfs while keeping the basic features.
Recently, Unionfs Version 2.x series begin taking some of the same
approaches to aufs1's.
Unionfs is being developed by Professor Erez Zadok at Stony Brook
University and his team.

This version of AUFS, aufs2 has several purposes.
- to be reviewed easily and widely.
- to make the source files simpler and smaller by dropping several
  original features.

Through this work, I found some bad things in aufs1 source code and
fixed them. Some of the dropped features will be reverted in the future,
but not all I'm afraid.
Aufs2 supports linux-2.6.27 and later. If you want older kernel version
support, try aufs1 from CVS on SourceForge.

Note: it becomes clear that "Aufs was rejected. Let's give it up."
According to Christoph Hellwig, linux rejects all union-type filesystems
but UnionMount.
<http://marc.info/?l=linux-kernel&m=123938533724484&w=2>


1. Features
----------------------------------------
- unite several directories into a single virtual filesystem. The member
  directory is called as a branch.
- you can specify the permission flags to the branch, which are 'readonly',
  'readwrite' and 'whiteout-able.'
- by upper writable branch, internal copyup and whiteout, files/dirs on
  readonly branch are modifiable logically.
- dynamic branch manipulation, add, del.
- etc...

Also there are many enhancements in aufs1, such as:
- readdir(3) in userspace.
- keep inode number by external inode number table
- keep the timestamps of file/dir in internal copyup operation
- seekable directory, supporting NFS readdir.
- support mmap(2) including /proc/PID/exe symlink, without page-copy
- whiteout is hardlinked in order to reduce the consumption of inodes
  on branch
- do not copyup, nor create a whiteout when it is unnecessary
- revert a single systemcall when an error occurs in aufs
- remount interface instead of ioctl
- maintain /etc/mtab by an external command, /sbin/mount.aufs.
- loopback mounted filesystem as a branch
- kernel thread for removing the dir who has a plenty of whiteouts
- support copyup sparse file (a file which has a 'hole' in it)
- default permission flags for branches
- selectable permission flags for ro branch, whether whiteout can
  exist or not
- export via NFS.
- support <sysfs>/fs/aufs and <debugfs>/aufs.
- support multiple writable branches, some policies to select one
  among multiple writable branches.
- a new semantics for link(2) and rename(2) to support multiple
  writable branches.
- no glibc changes are required.
- pseudo hardlink (hardlink over branches)
- allow a direct access manually to a file on branch, e.g. bypassing aufs.
  including NFS or remote filesystem branch.
- userspace wrapper for pathconf(3)/fpathconf(3) with _PC_LINK_MAX.
- and more...

Currently these features are dropped temporary from this version, aufs2.
See design/08plan.txt in detail.
- test only the highest one for the directory permission (dirperm1)
- show whiteout mode (shwh)
- copyup on open (coo=)
- nested mount, i.e. aufs as readonly no-whiteout branch of another aufs
  (robr)
- statistics of aufs thread (/sys/fs/aufs/stat)
- delegation mode (dlgt)
  a delegation of the internal branch access to support task I/O
  accounting, which also supports Linux Security Modules (LSM) mainly
  for Suse AppArmor.
- intent.open/create (file open in a single lookup)

Features or just an idea in the future (see also design/*.txt),
- reorder the branch index without del/re-add.
- permanent xino files for NFSD
- an option for refreshing the opened files after add/del branches
- 'move' policy for copy-up between two writable branches, after
  checking free space.
- O_DIRECT
- light version, without branch manipulation. (unnecessary?)
- copyup in userspace
- inotify in userspace
- readv/writev
- xattr, acl


2. Download
----------------------------------------
Kindly one of aufs user, the Center for Scientific Computing and Free
Software (C3SL), Federal University of Parana offered me a public GIT
tree space.
And there are several GIT servers. See http://aufs.sf.net in detail.

There are three GIT trees, aufs2-2.6, aufs2-standalone and aufs2-util.
While the aufs2-util is always necessary, you need either of aufs2-2.6
or aufs2-standalone.

The aufs2-2.6 tree includes the whole linux-2.6 GIT tree,
git://git.kernel.org/.../torvalds/linux-2.6.git.
And you cannot select CONFIG_AUFS_FS=m for this version, eg. you cannot
build aufs2 as an externel kernel module.
If you already have linux-2.6 GIT tree, you may want to pull and merge
the "aufs2" branch from this tree.

On the other hand, the aufs2-standalone tree has only aufs2 source files
and a necessary patch, and you can select CONFIG_AUFS_FS=m. In other
words, the aufs2-standalone tree is generated from aufs2-2.6 tree by,
- extract new files and modifications.
- generate some patch files from modifications.
- generate a ChangeLog file from git-log.
- commit the files newly and no log messages. this is not git-pull.

Both of aufs2-2.6 and aufs2-standalone trees have branches whose name is
in form of "aufs2" or "aufs2-xx" where "xx" represents the linux kernel
version, "linux-2.6.xx". For instance, "aufs2" (no -xx) for the latest
-rc version, and "aufs2-27" for linux-2.6.27,

o aufs2-2.6 tree
$ git clone --reference /your/linux-2.6/git/tree \
	http://git.c3sl.ufpr.br/pub/scm/aufs/aufs2-2.6.git \
	aufs2-2.6.git
- if you don't have linux-2.6 GIT tree, then remove "--reference ..."
$ cd aufs2-2.6.git
$ git checkout origin/aufs2-27

o aufs2-standalone tree
$ git clone http://git.c3sl.ufpr.br/pub/scm/aufs/aufs2-standalone.git \
	aufs2-standalone.git
$ cd aufs2-standalone.git
$ git checkout origin/aufs2-27

o aufs2-util tree
$ git clone http://git.c3sl.ufpr.br/pub/scm/aufs/aufs2-util.git \
	aufs2-util.git
$ cd aufs2-util.git
- no particular tag/branch currently.

o for advanced users
$ git clone git://git.kernel.org/.../torvalds/linux-2.6.git linux-2.6.git
  It will take very long time.

$ cd linux-2.6.git
$ git remote add aufs2 http://git.c3sl.ufpr.br/pub/scm/aufs/aufs2-2.6.git
$ git checkout -b aufs2-27 v2.6.27
$ git pull aufs2 aufs2-27
  It may take long time again.
  Once pulling completes, you've got linux-2.6.27 and aufs2 for it in a
  branch named aufs2-27, and you can configure and build it.

Or

$ git checkout -t -b aufs2 master
$ git pull aufs2 aufs2
  then you've got the latest linux kernel and the latest aufs2 in a
  branch named aufs2, and you can configure and build it.
  But aufs is released once a week, so you may meet a compilation error
  due to mismatching between the mainline and aufs2.

Or you may want build linux-2.6.xx.yy instead of linux-2.6.xx, then here
is an approach using linux-2.6-stable GIT tree.

$ cd linux-2.6.git/..
$ git clone -q --reference ./linux-2.6.git git://git.kernel.org/.../linux-2.6-stable.git \
	linux-2.6-stable.git
  It will take very long time.

$ cd linux-2.6-stable.git
$ git remote add aufs2 http://git.c3sl.ufpr.br/pub/scm/aufs/aufs2-2.6.git
$ git checkout -b aufs2-27.1 v2.6.27.1
$ git pull aufs2 aufs2-27
  then you've got linux-2.6.27.1 and aufs2 for 2.6.27 in a branch named
  aufs2-27.1, and you can configure and build it.
  But the changes made by v2.6.xx.yy may conflict with aufs2-xx, since
  aufs2-xx is for v2.6.xx only. In this case, you may find some patchces
  for v2.6.xx.yy in aufs2-standalone.git#aufs2-xx branch if someone else
  have ever requested me to support v2.6.xx.yy and I did it.

You can also check what was changed by pulling aufs2.
$ git diff v2.6.27.1..aufs2-27.1

If you want to check the changed files other than fs/aufs, then try this.
$ git diff v2.6.27.1..aufs2-27.1 |
> awk '
> /^diff / {new=1}
> /^diff.*aufs/ {new=0}
> new {print}
> '


3. Configuration and Compilation
----------------------------------------
Make sure you have git-checkout'ed the correct branch.

For aufs2-2.6 tree,
- enable CONFIG_EXPERIMENTAL and CONFIG_AUFS_FS.
- set other aufs configurations if necessary.

For aufs2-standalone tree,
There are several ways to build.

You may feel why aufs2-standalone.patch needs to export so many kernel
symbols. Because you selected aufs2-standalone tree instead of aufs2-2.6
tree. The number of necessary symbols to export essentially is zero.
All other symbols are for the external module.
If you don't like aufs2-standalone.patch, then try aufs2-2.6 tree.

1.
- apply ./aufs2-kbuild.patch to your kernel source files.
- apply ./aufs2-base.patch too.
- apply ./aufs2-standalone.patch too, if you have a plan to set
  CONFIG_AUFS_FS=m. otherwise you don't need ./aufs2-standalone.patch.
- copy ./{Documentation,fs,include} files to your kernel source tree.
- enable CONFIG_EXPERIMENTAL and CONFIG_AUFS_FS, you can select either
  =m or =y.
- and build your kernel as usual.
- install it and reboot your system.

2.
- module only (CONFIG_AUFS_FS=m).
- apply ./aufs2-base.patch to your kernel source files.
- apply ./aufs2-standalone.patch too.
- build your kernel and reboot.
- edit ./config.mk and set other aufs configurations if necessary.
  Note: You should read ./fs/aufs/Kconfig carefully which describes
  every aufs configurations.
- build the module by simple "make".
- you can specify ${KDIR} make variable which points to your kernel
  source tree.
- copy the build ./aufs.ko to /lib/modules/..., and run depmod -a (or
  reboot simply).
- no need to apply aufs2-kbuild.patch, nor copying source files to your
  kernel source tree.

And then,
- read README in aufs2-util, build and install it
- if you want to use readdir(3) in userspace or pathconf(3) wrapper,
  then run "make install_ulib" too. And refer to the aufs manual in
  detail.


4. Usage
----------------------------------------
At first, make sure aufs2-util are installed, and please read the aufs
manual, aufs.5 in aufs2-util.git tree.
$ man -l aufs.5

And then,
$ mkdir /tmp/rw /tmp/aufs
# mount -t aufs -o br=/tmp/rw:${HOME} none /tmp/aufs

Here is another example. The result is equivalent.
# mount -t aufs -o br=/tmp/rw=rw:${HOME}=ro none /tmp/aufs
  Or
# mount -t aufs -o br:/tmp/rw none /tmp/aufs
# mount -o remount,append:${HOME} /tmp/aufs

Then, you can see whole tree of your home dir through /tmp/aufs. If
you modify a file under /tmp/aufs, the one on your home directory is
not affected, instead the same named file will be newly created under
/tmp/rw. And all of your modification to a file will be applied to
the one under /tmp/rw. This is called the file based Copy on Write
(COW) method.
Aufs mount options are described in aufs.5.
If you run chroot or something and make your aufs as a root directory,
then you need to customize the shutdown script. See the aufs manual in
detail.

Additionally, there are some sample usages of aufs which are a
diskless system with network booting, and LiveCD over NFS.
See sample dir in CVS tree on SourceForge.


5. Contact
----------------------------------------
When you have any problems or strange behaviour in aufs, please let me
know with:
- /proc/mounts (instead of the output of mount(8))
- /sys/module/aufs/*
- /sys/fs/aufs/* (if you have them)
- /debug/aufs/* (if you have them)
- linux kernel version
  if your kernel is not plain, for example modified by distributor,
  the url where i can download its source is necessary too.
- aufs version which was printed at loading the module or booting the
  system, instead of the date you downloaded.
- configuration (define/undefine CONFIG_AUFS_xxx)
- kernel configuration or /proc/config.gz (if you have it)
- behaviour which you think to be incorrect
- actual operation, reproducible one is better
- mailto: aufs-users at lists.sourceforge.net

Usually, I don't watch the Public Areas(Bugs, Support Requests, Patches,
and Feature Requests) on SourceForge. Please join and write to
aufs-users ML.


6. Acknowledgements
----------------------------------------
Thanks to everyone who have tried and are using aufs, whoever
have reported a bug or any feedback.
Since Apr 2010, Tomas M (the author of Slax and Linux Live scripts) is
making "doubling" donations.
Unfortunately I cannot list all of the donators, but I really appriciate. See
<http://sourceforge.net/mailarchive/message.php?msg_name=4BC70596.4000507%40slax.org>
and
<http://www.slax.org/aufs.php>
in detail.

Especially donators:
Tomas Matejicek(slax.org) made a donation (much more than once).
Dai Itasaka made a donation (2007/8).
Chuck Smith made a donation (2008/4, 10 and 12).
Henk Schoneveld made a donation (2008/9).
Chih-Wei Huang, ASUS, CTC donated Eee PC 4G (2008/10).
Francois Dupoux made a donation (2008/11).
Bruno Cesar Ribas and Luis Carlos Erpen de Bona, C3SL serves public
aufs2 GIT tree (2009/2).
William Grant made a donation (2009/3).
Patrick Lane made a donation (2009/4).
The Mail Archive (mail-archive.com) made donations (2009/5).
Nippy Networks (Ed Wildgoose) made a donation (2009/7).
New Dream Network, LLC (www.dreamhost.com) made a donation (2009/11).

Thank you very much.
Donations are always, including future donations, very important and
helpful for me to keep on developing aufs.


7.
----------------------------------------
If you are an experienced user, no explanation is needed. Aufs is
just a linux filesystem.


Enjoy!

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