On Tue, Apr 17, 2007 at 12:56:24PM -0400, Shaya Potter wrote: > Bharata B Rao wrote: > > >No. foo is not visible. While looking for a file in a union mounted > >directory, the lookup starts from the topmost directory and proceeds > >downwards if the file isn't present the top layers. If a whiteout is > >found in any of the top layers, the lookup is abondoned and -ENOENT > >is removed. Thus until a whiteout exists in any upper layer for > >a corresponding file in the lower layer, the lower layer file remains > >hidden until the whiteout is removed. > > > >However in the case of dir-c containing foo, the foo(from dir-c) will > >become > >visible after union mounting dir-c on top of dir-b and dir-a. > > ok, so the major limitation of this approach is that the top most layer > has to either be, ext2, ext3 or tmpfs (in patch), and most likely not > NFS (assumption is that NFS has no conception of the whiteout type of > file). I haven't played with union mounts with NFS. Hence would let Jan answer this. However note that union mount provides a writable union only if the filesystem supports the notion of whiteouts. > One thing the unionfs people are doing w/ their ODF approach, is > within the ODF fs, they have a special inode that is the "whiteout" > inode, and when they create a whiteout, they just create a hardlink from > the dentry they want to whiteout to the "whiteout inode". could that be > a worthwhile approach instead of the whiteout file type? (i.e. many > file systems support the concept of a hard link). We we thinking something on similar lines as noted in our documentation. Right now we maintain one inode for every whiteout. We were planning to have a single whiteout inode and have all whiteout dentries point to this. But here again we were thinking of having every filesystem support this whiteout inode type. Anyway I will have a look at ODF from unionfs to see how this is done. > > I ask, because using union in a diskless environment. Imagine pxe > booting a kernel/initramfs and then using union to create a real root fs > (shared lower layer, private rw upper layer, ala live cds). Which > brings up a different point, with unionfs, one can pivot_root into it, > can one do the same for these "union mounts"? Don't know enough about > the VFS to know if this should "just work" or might be a problem. I would assume that it should 'just work'. But right now it is not working. Our code is not yet ready to correctly work with move mounts. Since pivot_root has semantics similar to move mounts, pivot_root is also not working. Also chroot to a union mount point is also not working atm. We will be working to get all these right. Regards, Bharata. - 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
