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 Mon, Sep 22, 2025 at 11:25:48PM +0000, Chris Laprise wrote:
> The overall procedure is:
> 
> 1. Get subvolume's Generation ID
> 
> 2. Read FIEMAP data
> 
> 3. Get subvolume's Generation ID again
> 
> 4. Check that the Generation ID hasn't changed: No match skips the file or
> raises an error

I'll let the btrfs developers answer this as it's clearly not about
XFS.

> 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.

> It is also understood that
> for some other applications FIEMAP would not be sufficient; however Wyng is
> not fetching or manipulating data at a low level.  Also, there is a lack of
> accuracy in the form of false positives, where unchanged data show up as
> deltas, but this only results in longer processing time not data corruption;
> false negatives are the only thing that must be avoided.

I think what you want/need is a way to look at the delta between two
reflinked files.  At least for XFS (and I'm pretty sure for btrfs as
well) the low-level data structures could provide this, but building an
actually safe interface to that is unfortunately hard.