linux-kernel - Re: [PATCH] tmpfs: support SEEK_DATA and SEEK

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Date:	Thu, 29 Nov 2012 12:42:51 +0800
From:	Jeff Liu <jeff.liu@...cle.com>
To:	Jim Meyering <jim@...ering.net>
CC:	Hugh Dickins <hughd@...gle.com>,
	Jaegeuk Hanse <jaegeuk.hanse@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Theodore Ts'o" <tytso@....edu>, Zheng Liu <wenqing.lz@...bao.com>,
	Paul Eggert <eggert@...ucla.edu>,
	Christoph Hellwig <hch@...radead.org>,
	Josef Bacik <josef@...hat.com>,
	Andi Kleen <andi@...stfloor.org>,
	Andreas Dilger <adilger@...ger.ca>,
	Dave Chinner <david@...morbit.com>,
	Marco Stornelli <marco.stornelli@...il.com>,
	Chris Mason <chris.mason@...ionio.com>,
	Sunil Mushran <sunil.mushran@...cle.com>,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [PATCH] tmpfs: support SEEK_DATA and SEEK_HOLE (reprise)

On 11/29/2012 12:15 PM, Jim Meyering wrote:
> Hugh Dickins wrote:
>> On Thu, 29 Nov 2012, Jaegeuk Hanse wrote:
> ...
>>> But this time in which scenario will use it?
>>
>> I was not very convinced by the grep argument from Jim and Paul:
>> that seemed to be grep holding on to a no-arbitrary-limits dogma,
>> at the expense of its users, causing an absurd line-length issue,
>> which use of SEEK_DATA happens to avoid in some cases.
>>
>> The cp of sparse files from Jeff and Dave was more convincing;
>> but I still didn't see why little old tmpfs needed to be ahead
>> of the pack.
>>
>> But at LinuxCon/Plumbers in San Diego in August, a more convincing
>> case was made: I was hoping you would not ask, because I did not take
>> notes, and cannot pass on the details - was it rpm building on tmpfs?
>> I was convinced enough to promise support on tmpfs when support on
>> ext4 goes in.
> 
> Re the cp-vs-sparse-file case, the current FIEMAP-based code in GNU
> cp is ugly and complicated enough that until recently it harbored a
> hard-to-reproduce data-corrupting bug[*].  Now that SEEK_DATA/SEEK_HOLE
> support work will work also for tmpfs and ext4, we can plan to remove
> the FIEMAP-based code in favor of a simpler SEEK_DATA/SEEK_HOLE-based
> implementation.
How do we teach du(1) to aware of the real disk footprint with Btrfs
clone or OCFS2 reflinked files if we remove the FIEMAP-based code?

How about if we still keep it there, and introduce SEEK_DATA/SEEK_HOLE
code to the extent-scan module which is dedicated to deal with sparse files?

Thanks,
-Jeff
> 
> With the rise of virtualization, copying sparse images efficiently
> (probably searching, too) is becoming more and more important.
> 
> So, yes, GNU cp will soon use this feature.
> 
> [*] https://plus.google.com/u/0/114228401647637059102/posts/FDV3JEaYsKD
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