| title: | Re PATCH 00 32 VFS based Union Mount V3 |
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On Tue, May 19, 2009 at 11:48:00AM +0200, Miklos Szeredi wrote:
On Mon, 18 May 2009, Jan Blunck wrote:
Here is another post of the VFS based union mount implementation.
Traditionally the mount operation is opaque, which means that the content of
the mount point, the directory where the file system is mounted on, is hidden
by the content of the mounted file systems root directory until the file
system is unmounted again. Unlike the traditional UNIX mount mechanism, that
hides the contents of the mount point, a union mount presents a view as if
both filesystems are merged together. Although only the topmost layer of the
mount stack can be altered, it appears as if transparent file system mounts
allow any file to be created, modified or deleted.
Most people know the concepts and features of union mounts from other
operating systems like Suns Translucent Filesystem, Plan9 or BSD. For an
in-depth review of union mounts and other unioning file systems, see:
rel="nofollow" lwn.net/Articles/324291/ lwn.net/Articles/324291/
rel="nofollow" lwn.net/Articles/325369/ lwn.net/Articles/325369/
rel="nofollow" lwn.net/Articles/327738/ lwn.net/Articles/327738/
Here are the key features of this implementation:
- completely VFS based
- does not change the namespace stacking
- directory listings have duplicate entries removed in the kernel
- writable unions: only the topmost file system layer may be writable
- writable unions: new whiteout filetype handled inside the kernel
Major changes since last post:
- Updated the whiteout patches:
- DCACHE_WHITEOUT flag set on a negative dentry
- uses filetype instead of reserved inode number on EXT2
- Copy-up directories during lookup
- Implemented fallthru support for in-kernel readdir() as proposed by
Valerie Aurora (Henson)
Does this copy up directories persistently? If so, does this
implementation no longer supports union of all read-only branches?
As Jan said, readdir() of read-only unioned file systems works with a
tmpfs top layer. If you think about it, this is the exact equivalent
of the version of union mounts which used the in-kernel caching
approach - except that its better, because it reuses existing code
and caches between readdir() calls. Cool, huh?
Do you have ideas for how to do this better? I particularly would
like to be able to get rid of the tmpfs dentries when no one is
readdir()ing on that directory.
-VAL
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