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

[Jörn Engel]

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.

Aye.

> 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.

Jörn

-- 
He who knows others is wise.
He who knows himself is enlightened.
-- Lao Tsu
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