Re: [PATCH 00/32] VFS based Union Mount (V3)

[Valerie Aurora <vaurora@redhat.com>]

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