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235 lines
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235 lines
9.2 KiB
Plaintext
Written by: Neil Brown
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Please see MAINTAINERS file for where to send questions.
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Overlay Filesystem
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==================
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This document describes a prototype for a new approach to providing
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overlay-filesystem functionality in Linux (sometimes referred to as
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union-filesystems). An overlay-filesystem tries to present a
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filesystem which is the result over overlaying one filesystem on top
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of the other.
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The result will inevitably fail to look exactly like a normal
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filesystem for various technical reasons. The expectation is that
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many use cases will be able to ignore these differences.
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This approach is 'hybrid' because the objects that appear in the
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filesystem do not all appear to belong to that filesystem. In many
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cases an object accessed in the union will be indistinguishable
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from accessing the corresponding object from the original filesystem.
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This is most obvious from the 'st_dev' field returned by stat(2).
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While directories will report an st_dev from the overlay-filesystem,
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all non-directory objects will report an st_dev from the lower or
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upper filesystem that is providing the object. Similarly st_ino will
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only be unique when combined with st_dev, and both of these can change
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over the lifetime of a non-directory object. Many applications and
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tools ignore these values and will not be affected.
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Upper and Lower
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---------------
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An overlay filesystem combines two filesystems - an 'upper' filesystem
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and a 'lower' filesystem. When a name exists in both filesystems, the
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object in the 'upper' filesystem is visible while the object in the
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'lower' filesystem is either hidden or, in the case of directories,
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merged with the 'upper' object.
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It would be more correct to refer to an upper and lower 'directory
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tree' rather than 'filesystem' as it is quite possible for both
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directory trees to be in the same filesystem and there is no
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requirement that the root of a filesystem be given for either upper or
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lower.
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The lower filesystem can be any filesystem supported by Linux and does
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not need to be writable. The lower filesystem can even be another
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overlayfs. The upper filesystem will normally be writable and if it
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is it must support the creation of trusted.* extended attributes, and
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must provide valid d_type in readdir responses, so NFS is not suitable.
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A read-only overlay of two read-only filesystems may use any
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filesystem type.
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Directories
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-----------
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Overlaying mainly involves directories. If a given name appears in both
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upper and lower filesystems and refers to a non-directory in either,
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then the lower object is hidden - the name refers only to the upper
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object.
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Where both upper and lower objects are directories, a merged directory
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is formed.
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At mount time, the two directories given as mount options "lowerdir" and
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"upperdir" are combined into a merged directory:
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mount -t overlay overlay -olowerdir=/lower,upperdir=/upper,\
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workdir=/work /merged
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The "workdir" needs to be an empty directory on the same filesystem
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as upperdir.
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Then whenever a lookup is requested in such a merged directory, the
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lookup is performed in each actual directory and the combined result
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is cached in the dentry belonging to the overlay filesystem. If both
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actual lookups find directories, both are stored and a merged
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directory is created, otherwise only one is stored: the upper if it
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exists, else the lower.
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Only the lists of names from directories are merged. Other content
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such as metadata and extended attributes are reported for the upper
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directory only. These attributes of the lower directory are hidden.
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whiteouts and opaque directories
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--------------------------------
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In order to support rm and rmdir without changing the lower
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filesystem, an overlay filesystem needs to record in the upper filesystem
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that files have been removed. This is done using whiteouts and opaque
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directories (non-directories are always opaque).
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A whiteout is created as a character device with 0/0 device number.
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When a whiteout is found in the upper level of a merged directory, any
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matching name in the lower level is ignored, and the whiteout itself
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is also hidden.
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A directory is made opaque by setting the xattr "trusted.overlay.opaque"
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to "y". Where the upper filesystem contains an opaque directory, any
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directory in the lower filesystem with the same name is ignored.
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readdir
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-------
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When a 'readdir' request is made on a merged directory, the upper and
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lower directories are each read and the name lists merged in the
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obvious way (upper is read first, then lower - entries that already
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exist are not re-added). This merged name list is cached in the
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'struct file' and so remains as long as the file is kept open. If the
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directory is opened and read by two processes at the same time, they
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will each have separate caches. A seekdir to the start of the
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directory (offset 0) followed by a readdir will cause the cache to be
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discarded and rebuilt.
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This means that changes to the merged directory do not appear while a
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directory is being read. This is unlikely to be noticed by many
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programs.
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seek offsets are assigned sequentially when the directories are read.
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Thus if
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- read part of a directory
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- remember an offset, and close the directory
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- re-open the directory some time later
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- seek to the remembered offset
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there may be little correlation between the old and new locations in
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the list of filenames, particularly if anything has changed in the
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directory.
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Readdir on directories that are not merged is simply handled by the
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underlying directory (upper or lower).
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renaming directories
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--------------------
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When renaming a directory that is on the lower layer or merged (i.e. the
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directory was not created on the upper layer to start with) overlayfs can
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handle it in two different ways:
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1. return EXDEV error: this error is returned by rename(2) when trying to
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move a file or directory across filesystem boundaries. Hence
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applications are usually prepared to hande this error (mv(1) for example
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recursively copies the directory tree). This is the default behavior.
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2. If the "redirect_dir" feature is enabled, then the directory will be
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copied up (but not the contents). Then the "trusted.overlay.redirect"
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extended attribute is set to the path of the original location from the
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root of the overlay. Finally the directory is moved to the new
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location.
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Non-directories
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---------------
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Objects that are not directories (files, symlinks, device-special
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files etc.) are presented either from the upper or lower filesystem as
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appropriate. When a file in the lower filesystem is accessed in a way
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the requires write-access, such as opening for write access, changing
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some metadata etc., the file is first copied from the lower filesystem
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to the upper filesystem (copy_up). Note that creating a hard-link
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also requires copy_up, though of course creation of a symlink does
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not.
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The copy_up may turn out to be unnecessary, for example if the file is
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opened for read-write but the data is not modified.
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The copy_up process first makes sure that the containing directory
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exists in the upper filesystem - creating it and any parents as
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necessary. It then creates the object with the same metadata (owner,
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mode, mtime, symlink-target etc.) and then if the object is a file, the
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data is copied from the lower to the upper filesystem. Finally any
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extended attributes are copied up.
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Once the copy_up is complete, the overlay filesystem simply
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provides direct access to the newly created file in the upper
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filesystem - future operations on the file are barely noticed by the
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overlay filesystem (though an operation on the name of the file such as
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rename or unlink will of course be noticed and handled).
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Multiple lower layers
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---------------------
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Multiple lower layers can now be given using the the colon (":") as a
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separator character between the directory names. For example:
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mount -t overlay overlay -olowerdir=/lower1:/lower2:/lower3 /merged
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As the example shows, "upperdir=" and "workdir=" may be omitted. In
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that case the overlay will be read-only.
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The specified lower directories will be stacked beginning from the
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rightmost one and going left. In the above example lower1 will be the
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top, lower2 the middle and lower3 the bottom layer.
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Non-standard behavior
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---------------------
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The copy_up operation essentially creates a new, identical file and
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moves it over to the old name. The new file may be on a different
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filesystem, so both st_dev and st_ino of the file may change.
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Any open files referring to this inode will access the old data.
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If a file with multiple hard links is copied up, then this will
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"break" the link. Changes will not be propagated to other names
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referring to the same inode.
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Unless "redirect_dir" feature is enabled, rename(2) on a lower or merged
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directory will fail with EXDEV.
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Changes to underlying filesystems
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---------------------------------
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Offline changes, when the overlay is not mounted, are allowed to either
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the upper or the lower trees.
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Changes to the underlying filesystems while part of a mounted overlay
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filesystem are not allowed. If the underlying filesystem is changed,
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the behavior of the overlay is undefined, though it will not result in
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a crash or deadlock.
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Testsuite
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---------
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There's testsuite developed by David Howells at:
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git://git.infradead.org/users/dhowells/unionmount-testsuite.git
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Run as root:
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# cd unionmount-testsuite
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# ./run --ov
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