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

[Demi Marie Obenour <demiobenour@gmail.com>]

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On 10/3/25 03:44, Christoph Hellwig wrote:
> On Mon, Sep 29, 2025 at 07:55:22PM -0400, Demi Marie Obenour wrote:
>> Can two extents have the same offset on disk but different logical contents?
>> For XFS that seems impossible unless a realtime device is involved, which
>> is not the case here.
> Only with the RT device.  But again that is insider knowledge and not an
> API exposed to applications.  More importantly the offset can change
> underneath without any notice to the application.

Are there cases where the offset can change even if neither file is
written to and there is no RT device?

>> Is it easier if one requires the filesystem to be read-only?  Taking a
>> device-mapper snapshot (thin or CoW) before the backup is not too onerous,
>> at least if the filesystem is already on an LVM LV.
> At least that locks out other changes.  But it is a rather opaque setup.

It's indeed not great, but if it is set up by the OS installer the
snapshots could all be automated.  Is it at currently safe in this
case, provided that online fsck is disabled?
-- 
Sincerely,
Demi Marie Obenour (she/her/hers)

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