Re: [ANNOUNCEMENT PATCH COW] proof of concept impementation of cowlinks

From: Jörn Engel
Date: Thu May 13 2004 - 05:59:59 EST

On Wed, 12 May 2004 01:26:06 +0100, Jamie Lokier wrote:
> Jörn Engel wrote:
> > What about ino? I currently return 1, so diff remains fast without
> > any changes. If someone really needs the difference between inode 2
> > and 3, I would introduce a cstat() system call similar to lstat(),
> > which would return ino=2.
> >
> > Is this sane? Should it be reversed and cstat() return ino=1, while
> > stat returns ino=2? I can imagine that "tar -x" would create hard
> > links for every cowlink that "tar -c" saw, but I'm not sure yet.
> I think it should be reversed.


> One very useful application for cowlinks is for virtual machine (UML)
> and chroot jail setups, where an entire filesystem tree is copied
> perhaps hundreds of times on a single disk. I'm surprised we didn't
> think of this earlier, as it's potentially one of the most useful
> applications for cowlinks.
> In that scenario, cowlinks would save enormous amounts of storage and
> potentially save memory too. However to be useful at all, they'd need
> to have accurate POSIX semantics: that is, cowlinks must behave very
> much as a storage optimisation only.
> That means stat() should return ino==2.

Hmm, true. Up 'till now, that was done with disk images and block
based diffs/snapshots. Nice application.


He who knows others is wise.
He who knows himself is enlightened.
-- Lao Tsu
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at
Please read the FAQ at