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From: Andreas Dilger <adilger@dilger.ca>
To: David Howells <dhowells@redhat.com>
Cc: Christoph Hellwig <hch@lst.de>,
	Qu Wenruo <quwenruo.btrfs@gmx.com>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	Al Viro <viro@zeniv.linux.org.uk>,
	"Theodore Y. Ts'o" <tytso@mit.edu>,
	"Darrick J. Wong" <darrick.wong@oracle.com>,
	Chris Mason <clm@fb.com>, Josef Bacik <josef@toxicpanda.com>,
	David Sterba <dsterba@suse.com>,
	linux-ext4 <linux-ext4@vger.kernel.org>,
	linux-xfs <linux-xfs@vger.kernel.org>,
	linux-btrfs <linux-btrfs@vger.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: Problems with determining data presence by examining extents?
Date: Wed, 15 Jan 2020 15:11:51 -0700	[thread overview]
Message-ID: <7233E240-8EE5-4CD1-B8A4-A90925F51A1B@dilger.ca> (raw)
In-Reply-To: <23762.1579121702@warthog.procyon.org.uk>

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On Jan 15, 2020, at 1:55 PM, David Howells <dhowells@redhat.com> wrote:
> 
> Andreas Dilger <adilger@dilger.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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  reply	other threads:[~2020-01-15 22:12 UTC|newest]

Thread overview: 24+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-01-14 16:48 Problems with determining data presence by examining extents? David Howells
2020-01-14 22:49 ` Theodore Y. Ts'o
2020-01-15  3:54 ` Qu Wenruo
2020-01-15 12:46   ` Andreas Dilger
2020-01-15 13:10     ` Qu Wenruo
2020-01-15 13:31       ` Christoph Hellwig
2020-01-15 19:48         ` Andreas Dilger
2020-01-16 10:16           ` Christoph Hellwig
2020-01-15 20:55         ` David Howells
2020-01-15 22:11           ` Andreas Dilger [this message]
2020-01-15 23:09           ` David Howells
2020-01-26 18:19             ` Zygo Blaxell
2020-01-15 14:35       ` David Howells
2020-01-15 14:48         ` Christoph Hellwig
2020-01-15 14:59         ` David Howells
2020-01-16 10:13           ` Christoph Hellwig
2020-01-17 16:43           ` David Howells
2020-01-15 14:20   ` David Howells
2020-01-15  8:38 ` Christoph Hellwig
2020-01-15 13:50 ` David Howells
2020-01-15 14:05 ` David Howells
2020-01-15 14:24   ` Qu Wenruo
2020-01-15 14:50   ` David Howells
2020-01-15 14:15 ` David Howells

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