linux-kernel - Re: Problems with determining data presence by examining extents?

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Message-ID: <23762.1579121702@warthog.procyon.org.uk>
Date:   Wed, 15 Jan 2020 20:55:02 +0000
From:   David Howells <dhowells@...hat.com>
To:     Andreas Dilger <adilger@...ger.ca>
Cc:     dhowells@...hat.com, Christoph Hellwig <hch@....de>,
        Qu Wenruo <quwenruo.btrfs@....com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        "Theodore Y. Ts'o" <tytso@....edu>,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        Chris Mason <clm@...com>, Josef Bacik <josef@...icpanda.com>,
        David Sterba <dsterba@...e.com>,
        linux-ext4 <linux-ext4@...r.kernel.org>,
        linux-xfs <linux-xfs@...r.kernel.org>,
        linux-btrfs <linux-btrfs@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Problems with determining data presence by examining extents?

Andreas Dilger <adilger@...ger.ca> 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.

David