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Date:   Sun, 22 Jan 2023 23:17:49 -0800
From:   Christoph Hellwig <hch@infradead.org>
To:     Cerem Cem ASLAN <ceremcem@ceremcem.net>
Cc:     Btrfs BTRFS <linux-btrfs@vger.kernel.org>
Subject: Re: Will Btrfs have an official command to "uncow" existing files?
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On Sun, Jan 22, 2023 at 02:41:22PM +0300, Cerem Cem ASLAN wrote:
> Original post is here: https://www.spinics.net/lists/linux-btrfs/msg58055.html
> 
> The problem with the "chattr +C ..., move back and forth" approach is
> that the VM folder is about 300GB and I have ~100GB of free space,
> plus, I have multiple copies which will require that 300GB to
> re-transfer after deleting all previous snapshots (because there is no
> enough free space on those backup hard disks).
> 
> So, we really need to set the NoCow attribute for the existing files.
> 
> Should we currently use a separate partition for VMs and mount it with
> nodatacow option to avoid that issue?

So, Linux for a while now has the FALLOC_FL_UNSHARE_RANGE flag to
fallocate to unshare previously shared extents.  It still lacks an
implementation for btrfs, but it seems to be the interface that you
want.