228 lines
8.9 KiB
Plaintext
228 lines
8.9 KiB
Plaintext
Written by: Neil Brown
|
|
Please see MAINTAINERS file for where to send questions.
|
|
|
|
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, so NFS is not suitable.
|
|
|
|
A read-only overlay of two read-only filesystems may use any
|
|
filesystem type.
|
|
|
|
Directories
|
|
-----------
|
|
|
|
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
|
|
object.
|
|
|
|
Where both upper and lower objects are directories, a merged directory
|
|
is formed.
|
|
|
|
At mount time, the two directories given as mount options "lowerdir" and
|
|
"upperdir" are combined into a merged directory:
|
|
|
|
mount -t overlay overlay -olowerdir=/lower,upperdir=/upper,\
|
|
workdir=/work /merged
|
|
|
|
The "workdir" needs to be an empty directory on the same filesystem
|
|
as upperdir.
|
|
|
|
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).
|
|
|
|
A whiteout is created as a character device with 0/0 device number.
|
|
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).
|
|
|
|
|
|
Multiple lower layers
|
|
---------------------
|
|
|
|
Multiple lower layers can now be given using the the colon (":") as a
|
|
separator character between the directory names. For example:
|
|
|
|
mount -t overlay overlay -olowerdir=/lower1:/lower2:/lower3 /merged
|
|
|
|
As the example shows, "upperdir=" and "workdir=" may be omitted. In
|
|
that case the overlay will be read-only.
|
|
|
|
The specified lower directories will be stacked beginning from the
|
|
rightmost one and going left. In the above example lower1 will be the
|
|
top, lower2 the middle and lower3 the bottom layer.
|
|
|
|
|
|
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.
|
|
|
|
Testsuite
|
|
---------
|
|
|
|
There's testsuite developed by David Howells at:
|
|
|
|
git://git.infradead.org/users/dhowells/unionmount-testsuite.git
|
|
|
|
Run as root:
|
|
|
|
# cd unionmount-testsuite
|
|
# ./run --ov
|