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Date:   Wed, 15 Jan 2020 23:09:03 +0000
From:   David Howells <>
To:     Andreas Dilger <>
Cc:, Christoph Hellwig <>,
        Qu Wenruo <>,
        linux-fsdevel <>,
        Al Viro <>,
        "Theodore Y. Ts'o" <>,
        "Darrick J. Wong" <>,
        Chris Mason <>, Josef Bacik <>,
        David Sterba <>,
        linux-ext4 <>,
        linux-xfs <>,
        linux-btrfs <>,
        Linux Kernel Mailing List <>
Subject: Re: Problems with determining data presence by examining extents?

Andreas Dilger <> wrote:

> > It would also have to say that blocks of zeros shouldn't be optimised away.
> I don't necessarily see that as a requirement, so long as the filesystem
> stores a "block" at that offset, but it could dedupe all zero-filled blocks
> to the same "zero block".  That still allows saving storage space, while
> keeping the semantics of "this block was written into the file" rather than
> "there is a hole at this offset".

Yeah, that's more what I was thinking of.  Provided I can find out that
something is present, it should be fine.


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