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Date:   Wed, 15 Jan 2020 15:11:51 -0700
From:   Andreas Dilger <adilger@...ger.ca>
To:     David Howells <dhowells@...hat.com>
Cc:     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?

On Jan 15, 2020, at 1:55 PM, David Howells <dhowells@...hat.com> wrote:
> 
> 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.

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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