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 system's 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 Sun's Translucent Filesystem, Plan9 or BSD. For an > > in-depth review of union mounts and other unioning file systems, see: > > > > http://lwn.net/Articles/324291/ > > http://lwn.net/Articles/325369/ > > http://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 it's 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 -- 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
