linux-kernel - Re: [PATCH][RFC] fast file mapping for loop

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Date:	Thu, 10 Jan 2008 09:42:06 +0100
From:	Jens Axboe <jens.axboe@...cle.com>
To:	linux-kernel@...r.kernel.org, chris.mason@...cle.com,
	linux-fsdevel@...r.kernel.org,
	Mikulas Patocka <mikulas@...ax.karlin.mff.cuni.cz>,
	"Bryn M. Reeves" <breeves@...hat.com>
Subject: Re: [PATCH][RFC] fast file mapping for loop

On Thu, Jan 10 2008, Jens Axboe wrote:
> On Wed, Jan 09 2008, Alasdair G Kergon wrote:
> > Here's the latest version of dm-loop, for comparison.
> > 
> > To try it out, 
> >   ln -s dmsetup dmlosetup
> > and supply similar basic parameters to losetup.
> > (using dmsetup version 1.02.11 or higher)
> 
> Why oh why does dm always insist to reinvent everything? That's bad
> enough in itself, but on top of that most of the extra stuff ends up
> being essentially unmaintained.
> 
> If we instead improve loop, everyone wins.
> 
> Sorry to sound a bit harsh, but sometimes it doesn't hurt to think a bit
> outside your own sandbox.

So I looked at the code - it seems you build a full extent of the blocks
in the file, filling holes as you go along. I initally did that as well,
but that is to slow to be usable in real life.

You also don't support sparse files, falling back to normal fs
read/write paths. Supporting sparse files properly is a must, people
generally don't want to prealloc a huge disk backing.

-- 
Jens Axboe

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