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SubjectRe: [PATCH 6/6 v6] overlay: overlay filesystem documentation
On Tue, 01 Mar 2011 13:36:51 +0100 Miklos Szeredi wrote:

> From: Neil Brown <>
> Document the overlay filesystem.
> Signed-off-by: Miklos Szeredi <>
> ---
> Documentation/filesystems/overlayfs.txt | 163 ++++++++++++++++++++++++++++++++
> 1 file changed, 163 insertions(+)
> Index: linux-2.6/Documentation/filesystems/overlayfs.txt
> ===================================================================
> --- /dev/null 1970-01-01 00:00:00.000000000 +0000
> +++ linux-2.6/Documentation/filesystems/overlayfs.txt 2011-03-01 12:18:20.000000000 +0100
> @@ -0,0 +1,163 @@
> +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
> +case 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 for the overlay-filesystem,
> +all non-directory objects will report an st_dev whichever of the
> +'lower' or 'upper' filesystem that is providing the object. Similarly

awkward sentence above. How is this?

While directories will report an st_dev for the overlay-filesystem,
all non-directory objects will report an st_dev from whichever of the
'lower' or 'upper' filesystem is providing the object.

or just:
from the lower or upper
filesystem that is providing the object.

> +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, at least for symbolic
> +links - so NFS is not suitable.
> +
> +A read-only overlay of two read-only filesystems may use any
> +filesystem type.
> +
> +Directories
> +-----------
> +
> +Overlaying mainly involved 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 are combined
> +into a merged directory. 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).
> +
> +The overlay filesystem uses extended attributes with a
> +"trusted.overlay." prefix to record these details.
> +
> +A whiteout is created as a symbolic link with target
> +"(overlay-whiteout)" and with xattr "trusted.overlay.whiteout" set to "y".
> +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 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).
> +
> +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. This is not yet enforced, but will be in
> +the future.
> --

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