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 9/29/25 04:49, Christoph Hellwig wrote:
> 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.

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.

>> 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.

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.
-- 
Sincerely,
Demi Marie Obenour (she/her/hers)

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