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

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Date:	Thu, 10 Jan 2008 14:03:24 +0100
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Chris Mason <chris.mason@...cle.com>
Cc:	Christoph Hellwig <hch@...radead.org>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH][RFC] fast file mapping for loop

On Thu, Jan 10 2008, Chris Mason wrote:
> On Thu, 10 Jan 2008 08:54:59 +0000
> Christoph Hellwig <hch@...radead.org> wrote:
> 
> > On Thu, Jan 10, 2008 at 09:44:57AM +0100, Jens Axboe wrote:
> > > > IMHO this shouldn't be done in the loop driver anyway.
> > > > Filesystems have their own effricient extent lookup trees (well,
> > > > at least xfs and btrfs do), and we should leverage that instead
> > > > of reinventing it.
> > > 
> > > Completely agree, it's just needed right now for this solution
> > > since all we have is a crappy bmap() interface to get at those
> > > mappings.
> > 
> > So let's fix the interface instead of piling crap ontop of it.  As I
> > said I think Peter has something to start with so let's beat on it
> > until we have something suitable.   If we aren't done by end of Feb
> > I'm happy to host a hackfest to get it sorted around the fs/storage
> > summit..
> > 
> 
> Ok, I've been meaning to break my extent_map code up, and this is a
> very good reason.  I'll work up a sample today based on Jens' code.

Great!

> The basic goals:
> 
> * Loop (swap) calls into the FS for each mapping. Any caching happens
> on the FS side.
> * The FS returns an extent, filling any holes

We don't want to fill holes for a read, but I guess that's a given?

> Swap would need to use an extra call early on for preallocation.
> 
> Step two is having a call back into the FS allow the FS to delay the
> bios until commit completion so that COW and delalloc blocks can be
> fully on disk when the bios are reported as done.  Jens, can you add
> some way to queue the bio completions up?

Sure, a function to save a completed bio and a function to execute
completions on those already stored?

-- 
Jens Axboe

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