Aufs -- Another Unionfs
Junjiro Okajima

# $Id: README,v 1.46 2007/04/30 05:48:24 sfjro Exp $


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.
Unionfs is being developed by Professor Erez Zadok at Stony Brook
University and his team.
If you don't know Unionfs, I recommend you to try and know it before
using aufs. Some terminology in aufs follows Unionfs's.

Bug reports (including my broken English), suggestions, comments
and donations are always welcome. Your bug report may help other users,
including future users. Especially the bug which doesn't follow unix/linux
filesystem's semantics is important.


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... see Unionfs in detail.

Also there are many enhancements in aufs, such like,,,
- safer and faster
- 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 shell script, /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.
- <sysfs>/fs/aufs.
- and more... see aufs manual in detail

Aufs is in still development stage, especially,,,
- 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.

(current work)
- refine xino and revalidate
- lockdep
- a delegation of the internal branch access to support task I/O
  accounting, which also supports Linux Security Modules (LSM) mainly
  for Suse AppArmor.
- nested mount, i.e. aufs as readonly no-whiteout branch of another aufs.

(just an idea)
- sample for kvm, xen, vmware and openvz? If a directory can be
  sharable between several Guest-OSes, to share readonly aufs branches
  will be an advantage.
- remount option copy/move between two branches. (unnecessary?)
- O_DIRECT (unnecessary?)
- light version, without branch manipulation. (unnecessary?)
- SMP, because I don't have such machine. But several users reported
  aufs is working fine on SMP machines.
- 64bit environment, because I don't have such machine.
- copyup in userspace
- xattr, acl
- disk quota (unnecessary?)
- and documentation


2. Download
----------------------------------------
CVS tree is in aufs project of SourceForge.
Here is simple instructions to get aufs source files. It is recommended to
refer to the document about CVS on SourceForge.
	$ mkdir aufs.wcvs
	$ cd aufs.wcvs
	$ cvs -d:pserver:anonymous@aufs.cvs.sourceforge.net:/cvsroot/aufs login
	(CVS password is empty)
	$ cvs -z3 -d:pserver:anonymous@aufs.cvs.sourceforge.net:/cvsroot/aufs co aufs

In order to update files after the first checkout.
	$ cd aufs.wcvs/aufs
	$ cvs update

In order to see what the difference between two versions (two dates) is,
	$ cd aufs.wcvs/aufs
	$ cvs diff -D20061212 -D20061219

I always try putting the stable version in CVS, so you can try CVS
instead of SourceForge File Release. And every changes are summarized
and reported to aufs-users at lists.sourceforge.net ML. It is recommended to
join this ML.


3. Configuration and Compilation
----------------------------------------
Aufs is being developed and tested on linux-2.6.16 and later.
You need the correct environment to build a kernel module. Generally
it is an environment which your kernel was built.
If you are unsure that you got the correct environment, then I
recommend you to built your kernel by yourself. If it succeeds, you've
got the correct environment.

Currently aufs configuration is written in ./Kconfig.in. You can build the
real ./fs/aufs/Kconfig file by 'make.' And follow the instructions which will
be produced. The built ./fs/aufs/Kconfig depends upon the kernel version.
	$ make -f local.mk kconfig
The local.mk searches your kernel build path by
	KDIR = /lib/modules/$(shell uname -r)/build
If you are cross-compiling the aufs module, try
	$ make KDIR=/your/kernel/build/path -f local.mk kconfig
If you compile aufs statically, the built Kconfig will help you.

Also you can use ./local.mk to compile aufs as a module by simply,
	$ make -f local.mk
or
	$ make KDIR=/your/kernel/build/path -f local.mk

The default configuration is written in ./local.mk too, they will work
for you in most cases. You can edit this file and change the
configuration for your aufs module.
If you don't want to change the ./local.mk, then create a new file
./priv_def.mk and write the definitions your aufs configuration. The
./local.mk includes ./priv_def.mk it it exists.

Currently, CONFIG_DEBUG_PROVE_LOCKING (in linux kernel) is not
supported since MAX_LOCKDEP_SUBCLASSES is too low for a stackable
filesystem. I am trying reducing the depth of lock subclasses.

When you test the performance, it is recommended to disable
CONFIG_AUFS_DEBUG. It is enabled by default for the feature test.


4. Usage
----------------------------------------
After 'make',
	$ man -l ./aufs.5
	# install -m 500 -p mount.aufs umount.aufs auplink aulchown /sbin (recommended)
	# insmod ./aufs.ko
	$ mkdir /tmp/rw /tmp/aufs
	# mount -t aufs -o dirs=/tmp/rw:${HOME}=ro none /tmp/aufs

Here is another example.

	# mount -t aufs -o br:/tmp/rw:${HOME}=ro none /tmp/aufs
	or
	# mount -t aufs -o br:/tmp/rw none /tmp/aufs
	# mount -o remount,append:${HOME}=ro /tmp/aufs

If you disable CONFIG_AUFS_COMPAT in your configuration, you can remove the
default branch permission '=ro.'

	# mount -t aufs -o br:/tmp/rw:${HOME} none /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, but the same named file will be newly created under
/tmp/rw. And all of your modification to the file will be applied to
the one under /tmp/rw. This is called Copy on Write (COW) method.
Aufs mount options are described in the generated ./aufs.5.

Additionally, there are some sample usages of aufs which are a
diskless system with network booting, and LiveCD over NFS.
See ./sample/diskless in detail.


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/fs/aufs/* (if you have them)
- linux kernel version
- AUFS_VERSION (defined in include/linux/aufs_type.h)
- configuration (define/undefine CONFIG_AUFS_xxx, or plain local.mk is
  used or not)
- phenomenon
- 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,
Feature Requests) on SourceForge. Please join and write to aufs-users ML.


6.
----------------------------------------
If you are an experienced user, no explanation is needed. Aufs is
just a linux filesystem module. take a glance at ./local.mk,
aufs.5, and Unionfs.


Enjoy!

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