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Date:   Wed, 15 Jan 2020 15:11:51 -0700
From:   Andreas Dilger <>
To:     David Howells <>
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?

On Jan 15, 2020, at 1:55 PM, David Howells <> wrote:
> Andreas Dilger <> wrote:
>> I think what is needed here is an fadvise/ioctl that tells the filesystem
>> "don't allocate blocks unless actually written" for that file.
> Yeah - and it would probably need to find its way onto disk so that its effect
> is persistent and visible to out-of-kernel tools.
> 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".

Cheers, Andreas

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