[Ocfs2-devel] [PATCH 1/3] fs: Document the reflink(2) system call.

[Joel Becker <Joel.Becker@oracle.com>]

On Mon, May 04, 2009 at 06:23:27PM -0400, Theodore Tso wrote:
> On Mon, May 04, 2009 at 02:18:54PM -0700, Joel Becker wrote:
> > 	More thinking.  It looks like we'll restrict reflink() to owners
> > or people with CAP_FCHOWN.  This prevents some quota DoS behavior.
> > 	We need to pre-charge all quota.  That means a reflink must be
> > charged the entire size of the file.  So, if I do:
> > 
> >   # dd if=/dev/zero bs=1M count=1 of=foo
> >   # reflink foo bar
> > 
> > I am now charged 2MB of quota, even though foo and bar share the same
> > 1MB of space.
> 
> Yep; but as long as you do this, why do you need CAP_FCHOWN?  

	Because the ownership doesn't change, and thus the person doing
the reflink is effectively setting ownership.

> Suppose Alice has a 1MB file, and Bob creates a reflink to it.  The
> reflink would be owned by Bob, and Bob would be charged the 1MB quota.
> This mirrors exactly what happens if Bob were to make a copy of the
> file, and we want to make the creation of reflink mirror a copy, right?

	It's more a link(2).  The ownership, permissions, and attributes
are identical to the original.

-- 

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Joel Becker
Principal Software Developer
Oracle
E-mail: joel.becker at oracle.com
Phone: (650) 506-8127