441 lines
19 KiB
Plaintext
441 lines
19 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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Overlay objects
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---------------
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The overlay filesystem approach is 'hybrid', because the objects that
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appear in the filesystem do not always appear to belong to that filesystem.
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In many 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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non-directory objects may report an st_dev from the lower filesystem 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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In the special case of all overlay layers on the same underlying
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filesystem, all objects will report an st_dev from the overlay
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filesystem and st_ino from the underlying filesystem. This will
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make the overlay mount more compliant with filesystem scanners and
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overlay objects will be distinguishable from the corresponding
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objects in the original filesystem.
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On 64bit systems, even if all overlay layers are not on the same
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underlying filesystem, the same compliant behavior could be achieved
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with the "xino" feature. The "xino" feature composes a unique object
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identifier from the real object st_ino and an underlying fsid index.
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If all underlying filesystems support NFS file handles and export file
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handles with 32bit inode number encoding (e.g. ext4), overlay filesystem
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will use the high inode number bits for fsid. Even when the underlying
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filesystem uses 64bit inode numbers, users can still enable the "xino"
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feature with the "-o xino=on" overlay mount option. That is useful for the
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case of underlying filesystems like xfs and tmpfs, which use 64bit inode
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numbers, but are very unlikely to use the high inode number bit.
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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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There are several ways to tune the "redirect_dir" feature.
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Kernel config options:
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- OVERLAY_FS_REDIRECT_DIR:
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If this is enabled, then redirect_dir is turned on by default.
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- OVERLAY_FS_REDIRECT_ALWAYS_FOLLOW:
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If this is enabled, then redirects are always followed by default. Enabling
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this results in a less secure configuration. Enable this option only when
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worried about backward compatibility with kernels that have the redirect_dir
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feature and follow redirects even if turned off.
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Module options (can also be changed through /sys/module/overlay/parameters/*):
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- "redirect_dir=BOOL":
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See OVERLAY_FS_REDIRECT_DIR kernel config option above.
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- "redirect_always_follow=BOOL":
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See OVERLAY_FS_REDIRECT_ALWAYS_FOLLOW kernel config option above.
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- "redirect_max=NUM":
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The maximum number of bytes in an absolute redirect (default is 256).
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Mount options:
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- "redirect_dir=on":
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Redirects are enabled.
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- "redirect_dir=follow":
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Redirects are not created, but followed.
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- "redirect_dir=off":
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Redirects are not created and only followed if "redirect_always_follow"
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feature is enabled in the kernel/module config.
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- "redirect_dir=nofollow":
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Redirects are not created and not followed (equivalent to "redirect_dir=off"
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if "redirect_always_follow" feature is not enabled).
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When the NFS export feature is enabled, every copied up directory is
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indexed by the file handle of the lower inode and a file handle of the
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upper directory is stored in a "trusted.overlay.upper" extended attribute
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on the index entry. On lookup of a merged directory, if the upper
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directory does not match the file handle stores in the index, that is an
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indication that multiple upper directories may be redirected to the same
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lower directory. In that case, lookup returns an error and warns about
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a possible inconsistency.
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Because lower layer redirects cannot be verified with the index, enabling
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NFS export support on an overlay filesystem with no upper layer requires
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turning off redirect follow (e.g. "redirect_dir=nofollow").
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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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Sharing and copying layers
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--------------------------
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Lower layers may be shared among several overlay mounts and that is indeed
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a very common practice. An overlay mount may use the same lower layer
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path as another overlay mount and it may use a lower layer path that is
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beneath or above the path of another overlay lower layer path.
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Using an upper layer path and/or a workdir path that are already used by
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another overlay mount is not allowed and may fail with EBUSY. Using
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partially overlapping paths is not allowed but will not fail with EBUSY.
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If files are accessed from two overlayfs mounts which share or overlap the
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upper layer and/or workdir path the behavior of the overlay is undefined,
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though it will not result in a crash or deadlock.
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Mounting an overlay using an upper layer path, where the upper layer path
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was previously used by another mounted overlay in combination with a
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different lower layer path, is allowed, unless the "inodes index" feature
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is enabled.
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With the "inodes index" feature, on the first time mount, an NFS file
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handle of the lower layer root directory, along with the UUID of the lower
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filesystem, are encoded and stored in the "trusted.overlay.origin" extended
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attribute on the upper layer root directory. On subsequent mount attempts,
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the lower root directory file handle and lower filesystem UUID are compared
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to the stored origin in upper root directory. On failure to verify the
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lower root origin, mount will fail with ESTALE. An overlayfs mount with
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"inodes index" enabled will fail with EOPNOTSUPP if the lower filesystem
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does not support NFS export, lower filesystem does not have a valid UUID or
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if the upper filesystem does not support extended attributes.
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It is quite a common practice to copy overlay layers to a different
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directory tree on the same or different underlying filesystem, and even
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to a different machine. With the "inodes index" feature, trying to mount
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the copied layers will fail the verification of the lower root file handle.
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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. Any open files referring to this inode
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will access the old data.
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The new file may be on a different filesystem, so both st_dev and st_ino
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of the real file may change. The values of st_dev and st_ino returned by
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stat(2) on an overlay object are often not the same as the real file
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stat(2) values to prevent the values from changing on copy_up.
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Unless "xino" feature is enabled, when overlay layers are not all on the
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same underlying filesystem, the value of st_dev may be different for two
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non-directory objects in the same overlay filesystem and the value of
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st_ino for directory objects may be non persistent and could change even
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while the overlay filesystem is still mounted.
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Unless "inode index" feature is enabled, if a file with multiple hard
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links is copied up, then this will "break" the link. Changes will not be
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propagated to other names 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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When the overlay NFS export feature is enabled, overlay filesystems
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behavior on offline changes of the underlying lower layer is different
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than the behavior when NFS export is disabled.
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On every copy_up, an NFS file handle of the lower inode, along with the
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UUID of the lower filesystem, are encoded and stored in an extended
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attribute "trusted.overlay.origin" on the upper inode.
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When the NFS export feature is enabled, a lookup of a merged directory,
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that found a lower directory at the lookup path or at the path pointed
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to by the "trusted.overlay.redirect" extended attribute, will verify
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that the found lower directory file handle and lower filesystem UUID
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match the origin file handle that was stored at copy_up time. If a
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found lower directory does not match the stored origin, that directory
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will not be merged with the upper directory.
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NFS export
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----------
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When the underlying filesystems supports NFS export and the "nfs_export"
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feature is enabled, an overlay filesystem may be exported to NFS.
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With the "nfs_export" feature, on copy_up of any lower object, an index
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entry is created under the index directory. The index entry name is the
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hexadecimal representation of the copy up origin file handle. For a
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non-directory object, the index entry is a hard link to the upper inode.
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For a directory object, the index entry has an extended attribute
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"trusted.overlay.upper" with an encoded file handle of the upper
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directory inode.
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When encoding a file handle from an overlay filesystem object, the
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following rules apply:
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1. For a non-upper object, encode a lower file handle from lower inode
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2. For an indexed object, encode a lower file handle from copy_up origin
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3. For a pure-upper object and for an existing non-indexed upper object,
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encode an upper file handle from upper inode
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The encoded overlay file handle includes:
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- Header including path type information (e.g. lower/upper)
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- UUID of the underlying filesystem
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- Underlying filesystem encoding of underlying inode
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This encoding format is identical to the encoding format file handles that
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are stored in extended attribute "trusted.overlay.origin".
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When decoding an overlay file handle, the following steps are followed:
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1. Find underlying layer by UUID and path type information.
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2. Decode the underlying filesystem file handle to underlying dentry.
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3. For a lower file handle, lookup the handle in index directory by name.
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4. If a whiteout is found in index, return ESTALE. This represents an
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overlay object that was deleted after its file handle was encoded.
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5. For a non-directory, instantiate a disconnected overlay dentry from the
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decoded underlying dentry, the path type and index inode, if found.
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6. For a directory, use the connected underlying decoded dentry, path type
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and index, to lookup a connected overlay dentry.
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Decoding a non-directory file handle may return a disconnected dentry.
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copy_up of that disconnected dentry will create an upper index entry with
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no upper alias.
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When overlay filesystem has multiple lower layers, a middle layer
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directory may have a "redirect" to lower directory. Because middle layer
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"redirects" are not indexed, a lower file handle that was encoded from the
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"redirect" origin directory, cannot be used to find the middle or upper
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layer directory. Similarly, a lower file handle that was encoded from a
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descendant of the "redirect" origin directory, cannot be used to
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reconstruct a connected overlay path. To mitigate the cases of
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directories that cannot be decoded from a lower file handle, these
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directories are copied up on encode and encoded as an upper file handle.
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On an overlay filesystem with no upper layer this mitigation cannot be
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used NFS export in this setup requires turning off redirect follow (e.g.
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"redirect_dir=nofollow").
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The overlay filesystem does not support non-directory connectable file
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handles, so exporting with the 'subtree_check' exportfs configuration will
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cause failures to lookup files over NFS.
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When the NFS export feature is enabled, all directory index entries are
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verified on mount time to check that upper file handles are not stale.
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This verification may cause significant overhead in some cases.
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Testsuite
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---------
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There's a testsuite originally developed by David Howells and currently
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maintained by Amir Goldstein at:
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https://github.com/amir73il/unionmount-testsuite.git
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Run as root:
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# cd unionmount-testsuite
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# ./run --ov --verify
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