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 Sun, Sep 21, 2025 at 08:07:10PM -0400, Demi Marie Obenour wrote:
> Wyng Backup (https://codeberg.org/tasket/wyng-backup) relies on FIEMAP
> to determine which parts of a file have not changed since it was last
> backed up.  Specifically, the output of filefrag -v is passed to sort and
> then to uniq, and differences between the outputs for the file and
> the previous version (a reflink copy) determine what gets backed up.
> 
> Is this safe under BTRFS, or can it result in data loss due to data
> not being backed up that should be?  In other words, can it result
> in data being considered unchanged when it really is?

This is not safe with any file system.  FIEMAP is purely a debugging
interface, and the output may or may not correspond to physical
block numbers.  E.g. for btrfs it points to virtual space, for XFS
it can point to the RT device.  It also is racy against file I/O.

The idea of that tool looks nice, but without a proper kernel interface
to look at the relationship between two files in a way that is locked
against I/O it is fundamentally unsafe.