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 04:50, 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.


I should have been clearer: this would be operating on an
unmounted filesystem, most likely obtained via a device-mapper
snapshot of the underlying block device.
-- 
Sincerely,
Demi Marie Obenour (she/her/hers)

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