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 29, 2025 at 09:34:32PM -0400, Demi Marie Obenour wrote:
> On 9/29/25 04:50, Christoph Hellwig wrote:
> > On Mon, Sep 22, 2025 at 01:54:56PM -0400, Demi Marie Obenour wrote:
> >> This leaves the question of whether the needed information is in the
> >> filesystem metadata.  If so, xfsprogs and/or btrfsprogs could obtain
> >> it from a block-layer snapshot offline without needing kernel changes.
> >> Otherwise, kernel changes will be needed.  I don't know if the changes
> >> to the userspace tools will be accepted, though.  Until then, btrfs
> >> send/receive will be the only way to efficiently back up a BTRFS
> >> filesystem, and XFS will only be able to be efficiently backed up
> >> at the block level.
> > 
> > Using userspace tools that poke at the block-level mapping is
> > fundamentally unsafe because it is not synchronized with the file
> > system.
> 
> Is it unsafe even if the filesystem is not mounted?

Well, if the file system is not mounted whoever pokes at it is obviously
in control.  And needs full understanding of the on-disk structures.