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Date:   Wed, 16 Mar 2022 15:04:26 -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 Wed, Mar 16, 2022 at 02:31:34PM -0400, Phillip Susi wrote:
> 
> Andrei Borzenkov <arvidjaar@gmail.com> writes:
> 
> > btrfs manages space in variable size extents. If you change 999 bytes in
> > 1000 bytes extent, original extent remains allocated because 1 byte is
> > still referenced. So actual space consumption is now 1999 bytes.
> 
> Huh?  You can't really do that because the page cache manages files in
> 4k pages.

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.

In theory you could create a 4098-byte file, then make reflinks from that
file to two other files covering the last 1 and last 2 bytes; however,
that's disallowed in the kernel to make sure that assorted dedupe data
leak shenanigans with shared reflinks that don't all end at the same
byte in the page can't ever happen.