+++ /dev/null
-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
-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, 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:
-
- mount -t overlayfs overlayfs -olowerdir=/lower,upperdir=/upper /overlay
-
-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 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 is 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 vaid 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 inconsitent 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.
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