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    Subject[PATCH 6/6 v6] overlay: overlay filesystem documentation
    From: Neil Brown <neilb@suse.de>

    Document the overlay filesystem.

    Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
    ---
    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 <neilb@suse.de>
    +
    +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
    +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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