To: debian-live at lists.debian.org
Subject: Replacing aufs with overlayfs
From: Ben Hutchings
Date: Tue, 09 Dec 2014

The overlayfs union filesystem was included in Linux 3.18. I am assuming that this will cover the needs of Debian live systems, so I've
dropped aufs from the Debian packaging. This is currently for experimental only, but you can expect this change to appear in unstable
shortly after the jessie release.

Please try the Linux 3.18 packages from experimental (they're not there yet, but should be soon) and check that overlayfs does what you need.

J. R. Okajima
Dec 8, 2014

Now linux-3.18 is released (and aufs3.18 too).
At the end of this year, all aufs versions before aufs3.14 will become obsolete and go to the unsupport-stage.

o news
- XATTR/EA is supported, still in testing.
Several new branch attributes are introduced.
For details, refer to linux/Documentations/filesystem/aufs/design/06xattr.txt and the manual in aufs-util.git.
- new mount options,"acl" and"noacl", are introduced.

Written by: Neil Brown

Overlay Filesystem
==================

This document describes a prototype for a new approach to providing overlay-filesystem functionality in Linux (sometimes referred to as union-filesystems).
An overlay-filesystem tries to present a filesystem which is the result over overlaying one filesystem on top of the other.

The result will inevitably fail to look exactly like a normal filesystem for various technical reasons.
The expectation is that many use cases will be able to ignore these differences.

This approach is 'hybrid' because the objects that appear in the filesystem do not all appear to belong to that filesystem.
In many cases an object accessed in the union will be indistinguishable from accessing the corresponding object from the original filesystem.
This is most obvious from the 'st_dev' field returned by stat(2).

While directories will report an st_dev from the overlay-filesystem,
all non-directory objects will report an st_dev from the lower or upper filesystem that is providing the object.
Similarly st_ino will only be unique when combined with st_dev, and both of these can change over the lifetime of a non-directory object.
Many applications and tools ignore these values and will not be affected.


Upper and Lower
---------------

An overlay filesystem combines two filesystems - an 'upper' filesystem and a 'lower' filesystem.
When a name exists in both filesystems, the object in the 'upper' filesystem is visible while the object in the 'lower' filesystem is either hidden
or, in the case of directories, merged with the 'upper' object.

It would be more correct to refer to an upper and lower 'directory tree' rather than 'filesystem' as it is quite possible
for both directory trees to be in the same filesystem and there is no requirement that the root of a filesystem be given for either upper or lower.

The lower filesystem can be any filesystem supported by Linux and does not need to be writable.
The lower filesystem can even be another overlayfs. The upper filesystem will normally be writable
and if it is it must support the creation of trusted.* extended attributes, and must provide valid d_type in readdir responses, so NFS is not suitable.

A read-only overlay of two read-only filesystems may use any filesystem type.


Directories
-----------

Overlaying mainly involves directories. If a given name appears in both upper and lower filesystems and refers to a non-directory in either,
then the lower object is hidden - the name refers only to the upper object.

Where both upper and lower objects are directories, a merged directory is formed.

At mount time, the two directories given as mount options"lowerdir" and"upperdir" are combined into a merged directory:

mount -t overlay overlay -olowerdir=/lower,upperdir=/upper, workdir=/work /merged

The"workdir" needs to be an empty directory on the same filesystem as upperdir.

Then whenever a lookup is requested in such a merged directory, the lookup is performed in each actual directory
and the combined result is cached in the dentry belonging to the overlay filesystem.
If both actual lookups find directories, both are stored and a merged directory is created, otherwise only one is stored: the upper if it exists, else the lower.

Only the lists of names from directories are merged. Other content such as metadata and extended attributes are reported for the upper directory only.
These attributes of the lower directory are hidden.


whiteouts and opaque directories
--------------------------------

In order to support rm and rmdir without changing the lower filesystem, an overlay filesystem needs to record in the upper filesystem that files have been removed.
This is done using whiteouts and opaque directories (non-directories are always opaque).

A whiteout is created as a character device with 0/0 device number.
When a whiteout is found in the upper level of a merged directory, any matching name in the lower level is ignored, and the whiteout itself is also hidden.

A directory is made opaque by setting the xattr"trusted.overlay.opaque" to"y".
Where the upper filesystem contains an opaque directory, any directory in the lower filesystem with the same name is ignored.


readdir
-------

When a 'readdir' request is made on a merged directory, the upper and lower directories are each read and the name lists merged in the
obvious way (upper is read first, then lower - entries that already exist are not re-added).
This merged name list is cached in the 'struct file' and so remains as long as the file is kept open.
If the directory is opened and read by two processes at the same time, they will each have separate caches.
A seekdir to the start of the directory (offset 0) followed by a readdir will cause the cache to be discarded and rebuilt.

This means that changes to the merged directory do not appear while a directory is being read.
This is unlikely to be noticed by many programs.

seek offsets are assigned sequentially when the directories are read.
Thus if
- read part of a directory
- remember an offset, and close the directory
- re-open the directory some time later
- seek to the remembered offset

there may be little correlation between the old and new locations in the list of filenames, particularly if anything has changed in the directory.

Readdir on directories that are not merged is simply handled by the underlying directory (upper or lower).


Non-directories
---------------

Objects that are not directories (files, symlinks, device-special files etc.) are presented either from the upper or lower filesystem as appropriate.
When a file in the lower filesystem is accessed in a way the requires write-access, such as opening for write access, changing
some metadata etc., the file is first copied from the lower filesystem to the upper filesystem (copy_up).
Note that creating a hard-link also requires copy_up, though of course creation of a symlink does not.

The copy_up may turn out to be unnecessary, for example if the file is opened for read-write but the data is not modified.

The copy_up process first makes sure that the containing directory exists in the upper filesystem - creating it and any parents as necessary.
It then creates the object with the same metadata (owner, mode, mtime, symlink-target etc.) and then if the object is a file, the
data is copied from the lower to the upper filesystem. Finally any extended attributes are copied up.

Once the copy_up is complete, the overlay filesystem simply provides direct access to the newly created file in the upper
filesystem - future operations on the file are barely noticed by the overlay filesystem (though an operation on the name of the file such as
rename or unlink will of course be noticed and handled).


Non-standard behavior
---------------------

The copy_up operation essentially creates a new, identical file and moves it over to the old name.
The new file may be on a different filesystem, so both st_dev and st_ino of the file may change.

Any open files referring to this inode will access the old data and metadata.
Similarly any file locks obtained before copy_up will not apply to the copied up file.

On a file opened with O_RDONLY fchmod(2), fchown(2), futimesat(2) and fsetxattr(2) will fail with EROFS.

If a file with multiple hard links is copied up, then this will"break" the link.
Changes will not be propagated to other names referring to the same inode.

Symlinks in /proc/PID/ and /proc/PID/fd which point to a non-directory object in overlayfs will not contain valid absolute paths,
only relative paths leading up to the filesystem's root. This will be fixed in the future.

Some operations are not atomic, for example a crash during copy_up or rename will leave the filesystem in an inconsistent state.
This will be addressed in the future.


Changes to underlying filesystems
---------------------------------

Offline changes, when the overlay is not mounted, are allowed to either the upper or the lower trees.

Changes to the underlying filesystems while part of a mounted overlay filesystem are not allowed.
If the underlying filesystem is changed, the behavior of the overlay is undefined, though it will not result in a crash or deadlock.