Re: Can the output of FIEMAP on BTRFS be used to check if a file and its reflink copy might have diverged?

[Christoph Hellwig <hch@infradead.org>]

On Sat, Oct 04, 2025 at 01:43:32AM +0000, Chris Laprise wrote:
> > > I should clarify that in this application we are not interested in physical
> > > mappings, but the logical representation of data.
> > 
> > And that's not what FIEMAP provides.
> 
> Its actually what FIEMAP provides: the reported 'logical offsets' are
> intra-file offsets mapped to extent addresses which are labeled 'physical'
> (in FIEMAP only) as a historical holdover in Btrfs.  From the "BTRFS: The
> Linux B-Tree Filesystem" paper [1] and developer reference [2]:

FIEMAP is NOT an application tool but a provider out debug output.
The target is supposed to be physical addresses, but as you've
noticed the interpretation varies widely.

If you use it, you will eventually lose data.  DONT do it, and do not
blame the file systems if it eventually happens.  If you want to be
known for a backup tools that loses data, go ahead.  But don't blame it
on anyone else.

And please, don't give me a reading on FIEMAP.  I've been involved with
it from the very beginning.  Reading out the documentation feels rather
patronizing.