On Fri, Dec 08, 2017 at 10:15:07AM +0200, Amir Goldstein wrote: > On Fri, Dec 8, 2017 at 2:13 AM, Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote: > > On Fri, Dec 08, 2017 at 10:59:22AM +1100, Dave Chinner wrote: > > > >> > 3. if multi nested looped fs is important (not really) then loop/nbd will > >> > need to know if its file is on a looped fs and propagate nesting level > >> > to ext4 > >> > >> This functionality is definitely used and needs to be supported by > >> the annotations. > > > > FWIW, in addition to loop, there's md. > > Do you mean md can sit on a file directly without loop/nbd? > I am referring to loop/nbd, because those are the only ones I know > of that can be used to nest (non stackable) fs over fs. The problem is one of stacked completions, not loop/nbd devices. The loop and nbd devices are just the simplest way to stack completions. MD can sit on MD and other devices in complex stacks, so if there's completion in MD, we have the same completion layering problem. i.e. this is *not specifically a filesystem problem*. This is a completion stacking problem, and lots of different layers in the storage stack use completions and can be layered in arbitrary orders. MD and DM are jsut two more virtual block devices that can stack in random orders. Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx
