From: Josef "Jeff" Sipek <jsipek@xxxxxxxxxxxxx> This patch contains documentation for Unionfs. You will find several files outlining basic unification concepts and rename semantics. Signed-off-by: Josef "Jeff" Sipek <jsipek@xxxxxxxxxxxxx> Signed-off-by: David Quigley <dquigley@xxxxxxxxxxxxxxxxx> Signed-off-by: Erez Zadok <ezk@xxxxxxxxxxxxx> --- 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. ufs.txt - info on the ufs filesystem. +unionfs/ + - info on the unionfs filesystem v9fs.txt - v9fs is a Unix implementation of the Plan 9 9p remote fs protocol. vfat.txt 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 @@ +00-INDEX + - this file. +concepts.txt + - A brief introduction of concepts +rename.txt + - Information regarding rename operations +usage.txt + - 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. + + +Whiteouts: +========== + +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: + +./b0/ +./b1/ +./b1/foo + +The unified view would simply be: + +./union/ +./union/foo + +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. + +./b0/ +./b0/.wh.foo +./b1/ +./b1/foo + +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. + + +Copyup: +======= + +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: + +dirs=branch[=ro|=rw][:...] +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. + +Syntax: +dirs=/branch1[=ro|=rw]:/branch2[=ro|=rw]:...:/branchN[=ro|=rw] + +Example: +dirs=/writable_branch=rw:/read-only_branch=ro + + +KNOWN ISSUES: +============= + +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 +RESULT IN DATA LOSS. + +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. + -- 1.4.3.3 - 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
