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Date:   Thu, 17 Mar 2022 18:06:04 -0400
From:   Zygo Blaxell <ce3g8jdj@umail.furryterror.org>
To:     Phillip Susi <phill@thesusis.net>
Cc:     Andrei Borzenkov <arvidjaar@gmail.com>,
        Jan Ziak <0xe2.0x9a.0x9b@gmail.com>,
        linux-btrfs@vger.kernel.org
Subject: Re: Btrfs autodefrag wrote 5TB in one day to a 0.5TB SSD without a
 measurable benefit
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On Thu, Mar 17, 2022 at 04:34:51PM -0400, Phillip Susi wrote:
> Zygo Blaxell <ce3g8jdj@umail.furryterror.org> writes:
> > You can get a 1-byte file reference if you make a reflink of the last
> > block of a 4097-byte file, or punch a hole in the first 4096 bytes of a
> > 4097-byte file.  This creates a file containing only a reference to the
> > last byte of the original extent.
> 
> So the inode only refers to one byte of the extent, but the extent is
> still always a multiple of 4k right?

Yes.

In theory, the on-disk format specifies extent locations and sizes in
bytes.  In practice, the kernel enforces that all the extent physical
boundaries be a multiple of the CPU page size (or multiples of _some_
CPU's page size, with the subpage patches).  On read, anything with a
logical length that isn't a multiple of 4K is zero-filled.