[PATCH 8/9] overlay: overlay filesystem documentation

From: Miklos Szeredi
Date: Wed Mar 13 2013 - 10:18:56 EST

From: Neil Brown <neilb@xxxxxxx>

Document the overlay filesystem.

Signed-off-by: Miklos Szeredi <mszeredi@xxxxxxx>
Documentation/filesystems/overlayfs.txt | 199 +++++++++++++++++++++++++++++++
2 files changed, 206 insertions(+)
create mode 100644 Documentation/filesystems/overlayfs.txt

diff --git a/Documentation/filesystems/overlayfs.txt b/Documentation/filesystems/overlayfs.txt
new file mode 100644
index 0000000..00dbab0
--- /dev/null
+++ b/Documentation/filesystems/overlayfs.txt
@@ -0,0 +1,199 @@
+Written by: Neil Brown <neilb@xxxxxxx>
+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
+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.
+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
+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.
+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
+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
+Readdir on directories that are not merged is simply handled by the
+underlying directory (upper or lower).
+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
+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.
index 9561658..9ea89b8 100644
@@ -5872,6 +5872,13 @@ F: drivers/scsi/osd/
F: include/scsi/osd_*
F: fs/exofs/

+M: Miklos Szeredi <miklos@xxxxxxxxxx>
+L: linux-fsdevel@xxxxxxxxxxxxxxx
+S: Supported
+F: fs/overlayfs/*
+F: Documentation/filesystems/overlayfs.txt
M: Christian Lamparter <chunkeey@xxxxxxxxxxxxxx>
L: linux-wireless@xxxxxxxxxxxxxxx

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