On Thu, Aug 05, 2010 at 01:13:55PM +0200, Miklos Szeredi wrote: > On Wed, 4 Aug 2010, Valerie Aurora wrote: > > > Another idea is to use an internal inode and make all fallthroughs be > > > hard links to that. > > > > > > I think the same would work for whiteouts as well. I don't like the > > > fact that whiteouts are invisible even when not mounted as part of a > > > union. > > > > I don't know if this helps, but I just wrote support for removing ext2 > > whiteouts and fallthrus using tune2fs and e2fsck. I think this does > > what people want from a "visible" whiteout feature without adding more > > complexity to the VFS. It also takes away all consideration of race > > conditions and dentry conversion that happens with online removal of > > whiteouts and fallthrus. > > > > What are your thoughts on what a visible whiteout/fallthru would look > > like? > > Best would be if it didn't need any modification to filesystems. All > this having to upgrade util-linux, e2fsprogs, having incompatible > filesystem features is a pain for users (just been through that). > > What we already have in most filesystems: > > - extended attributes, e.g. use the system.union.* namespace and > denote whiteouts and falltroughs with such an attribute > > - hard links to make sure a separate inode is not necessary for each > whiteout/fallthrough entry > > - some way for the user to easily identify such files when not > mounted as part of a union e.g. make it a symlink pointing to > "(deleted)" or whatever > > Later the extended attributes can also be used for other things like > e.g. chmod()/chown() only copying up metadata, not data, and > indicating that data is still found on the lower layers. Just a quick note to say that my explicit design was to do as much as possible in the VFS, except when adding a little support to the low-level fs would make it significantly faster, simpler, and more correct. I think for union mounts to perform moderately well, and to avoid namespace problems, we can't build it 100% out of existing file system parts like xattrs. However, I could be wrong and I will definitely give any other implementation serious consideration. -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
