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 9/22/25 12:48, Christoph Hellwig wrote:
>> 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.

Is it safe on XFS if there is no RT device and both files have been
fsync'd and are not modified by userspace?  I believe this is the case
here: reflinks are used to snapshot the files before they are looked at.

I am glad to know that BTRFS is unsafe and will report this.
-- 
Sincerely,
Demi Marie Obenour (she/her/hers)

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