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    Subject[PATCH 12/35] Unionfs: Documentation
    From: Josef "Jeff" Sipek <>

    This patch contains documentation for Unionfs. You will find several files
    outlining basic unification concepts and rename semantics.

    Signed-off-by: Josef "Jeff" Sipek <>
    Signed-off-by: David Quigley <>
    Signed-off-by: Erez Zadok <>
    Documentation/filesystems/00-INDEX | 2 +
    Documentation/filesystems/unionfs/00-INDEX | 8 +++
    Documentation/filesystems/unionfs/concepts.txt | 68 ++++++++++++++++++++++++
    Documentation/filesystems/unionfs/rename.txt | 31 +++++++++++
    Documentation/filesystems/unionfs/usage.txt | 41 ++++++++++++++
    5 files changed, 150 insertions(+), 0 deletions(-)

    diff --git a/Documentation/filesystems/00-INDEX b/Documentation/filesystems/00-INDEX
    index 4dc28cc..c737e3e 100644
    --- a/Documentation/filesystems/00-INDEX
    +++ b/Documentation/filesystems/00-INDEX
    @@ -82,6 +82,8 @@ udf.txt
    - info and mount options for the UDF filesystem.
    - info on the ufs filesystem.
    + - info on the unionfs filesystem
    - v9fs is a Unix implementation of the Plan 9 9p remote fs protocol.
    diff --git a/Documentation/filesystems/unionfs/00-INDEX b/Documentation/filesystems/unionfs/00-INDEX
    new file mode 100644
    index 0000000..fa87f83
    --- /dev/null
    +++ b/Documentation/filesystems/unionfs/00-INDEX
    @@ -0,0 +1,8 @@
    + - this file.
    + - A brief introduction of concepts
    + - Information regarding rename operations
    + - Usage & known limitations
    diff --git a/Documentation/filesystems/unionfs/concepts.txt b/Documentation/filesystems/unionfs/concepts.txt
    new file mode 100644
    index 0000000..0b9bcc9
    --- /dev/null
    +++ b/Documentation/filesystems/unionfs/concepts.txt
    @@ -0,0 +1,68 @@
    +This file describes the concepts needed by a namespace unification file system.
    +Branch Priority:
    +Each branch is assigned a unique priority - starting from 0 (highest priority).
    +No two branches can have the same priority.
    +Branch Mode:
    +Each branch is assigned a mode - read-write or read-only. This allows
    +directories on media mounted read-write to be used in a read-only manner.
    +A whiteout removes a file name from the namespace. Whiteouts are needed when
    +one attempts to remove a file on a read-only branch.
    +Suppose we have a two-branch union, where branch 0 is read-write and branch 1
    +is read-only. And a file 'foo' on branch 1:
    +The unified view would simply be:
    +Since 'foo' is stored on a read-only branch, it cannot be removed. A whiteout
    +is used to remove the name 'foo' from the unified namespace. Again, since
    +branch 1 is read-only, the whiteout cannot be created there. So, we try on a
    +higher priority (lower numerically) branch. And there we create the whiteout.
    +Later, when Unionfs traverses branches (due to lookup or readdir), it eliminate
    +'foo' from the namespace (as well as the whiteout itself.)
    +Duplicate Elimination:
    +It is possible for files on different branches to have the same name. Unionfs
    +then has to select which instance of the file to show to the user. Given the
    +fact that each branch has a priority associated with it, the simplest
    +solution is to take the instance from the highest priority (lowest numerical
    +value) and "hide" the others.
    +When a change is made to the contents of a file's data or meta-data, they
    +have to be stored somewhere. The best way is to create a copy of the
    +original file on a branch that is writable, and then redirect the write
    +though to this copy. The copy must be made on a higher priority branch so
    +that lookup & readdir return this newer "version" of the file rather than
    +the original (see duplicate elimination).
    diff --git a/Documentation/filesystems/unionfs/rename.txt b/Documentation/filesystems/unionfs/rename.txt
    new file mode 100644
    index 0000000..e20bb82
    --- /dev/null
    +++ b/Documentation/filesystems/unionfs/rename.txt
    @@ -0,0 +1,31 @@
    +Rename is a complex beast. The following table shows which rename(2) operations
    +should succeed and which should fail.
    +o: success
    +E: error (either unionfs or vfs)
    +X: EXDEV
    +none = file does not exist
    +file = file is a file
    +dir = file is a empty directory
    +child= file is a non-empty directory
    +wh = file is a directory containing only whiteouts; this makes it logically
    + empty
    + none file dir child wh
    +file o o E E E
    +dir o E o E o
    +child X E X E X
    +wh o E o E o
    +Renaming directories:
    +Whenever a empty (either physically or logically) directory is being renamed,
    +the following sequence of events should take place:
    +1) Remove whiteouts from both source and destination directory
    +2) Rename source to destination
    +3) Make destination opaque to prevent anything under it from showing up
    diff --git a/Documentation/filesystems/unionfs/usage.txt b/Documentation/filesystems/unionfs/usage.txt
    new file mode 100644
    index 0000000..8db9158
    --- /dev/null
    +++ b/Documentation/filesystems/unionfs/usage.txt
    @@ -0,0 +1,41 @@
    +Unionfs is a stackable unification file system, which can appear to merge the
    +contents of several directories (branches), while keeping their physical
    +content separate. Unionfs is useful for unified source tree management, merged
    +contents of split CD-ROM, merged separate software package directories, data
    +grids, and more. Unionfs allows any mix of read-only and read-write branches,
    +as well as insertion and deletion of branches anywhere in the fan-out. To
    +maintain unix semantics, Unionfs handles elimination of duplicates,
    +partial-error conditions, and more.
    +mount -t unionfs -o branch-option[,union-options[,...]] none MOUNTPOINT
    +The available branch-option for the mount command is:
    +specifies a separated list of which directories compose the union. Directories
    +that come earlier in the list have a higher precedence than those which come
    +later. Additionally, read-only or read-write permissions of the branch can be
    +specified by appending =ro or =rw (default) to each directory.
    +The NFS server returns -EACCES for read-only exports, instead of -EROFS. This
    +means we can't reliably detect a read-only NFS export.
    +Modifying a Unionfs branch directly, while the union is mounted is currently
    +unsupported. Any such change can cause Unionfs to oops, however it could even
    +Unionfs shouldn't use lookup_one_len on the underlying fs as it confuses
    +NFS. Currently, unionfs_lookup passes lookup intents to the lower
    +filesystem, this eliminates part of the problem. The remaining calls to
    +lookup_one_len may need to be changed to pass an intent.
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