From: Neil Brown <neilb@xxxxxxx> Document the overlay filesystem. Signed-off-by: Miklos Szeredi <mszeredi@xxxxxxx> --- 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 2010-09-20 15:17:15.000000000 +0200 @@ -0,0 +1,163 @@ +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 +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. Theoretically it could even be another +overlayfs, but this is not yet supported. The upper filesystem will +normally be writeable 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 -- -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
