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

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Date:	Thu, 29 Nov 2012 05:15:50 +0100
From:	Jim Meyering <jim@...ering.net>
To:	Hugh Dickins <hughd@...gle.com>
Cc:	Jaegeuk Hanse <jaegeuk.hanse@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Theodore Ts'o <tytso@....edu>,
	Zheng Liu <wenqing.lz@...bao.com>,
	Jeff liu <jeff.liu@...cle.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)

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.

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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