[PATCH v7 00/47] xfs: add reverse mapping support

[Artem Bityutskiy]

On Fri, 2016-08-05 at 20:49 +1000, Dave Chinner wrote:
> On Fri, Aug 05, 2016 at 10:01:14AM +0300, Artem Bityutskiy wrote:
> > 
> > On Fri, 2016-08-05 at 09:50 +1000, Dave Chinner wrote:
> > > 
> > > I'd much prefer that fiemap gives exact information about shared
> > > extents. FIEMAP is a diagnostic tool and as such we need it to
> > > accurately reflect the exact extent map of the inode being
> > > queried
> > > so we aren't mislead about the layout of the file during trouble
> > > shooting.
> > 
> > Hi Dave, you are right, and here is a side note:  we were using
> > FIEMAP
> > for optimizing image deployment in production, so it is a
> > diagnostic
> > tool and more.
> 
> Yay, data corruption ahoy!
> 
> Hasn't /anyone/ listened to the repeated statements from fs
> developers that FIEMAP is not a safe method of optimising data
> copying?

Yes, which is kind of sad from the user's perspective. We had issues
with XFS, not ext4, and tweaking preallocation helped.