linux-kernel - Re: [PATCH 3/6] direct-io: do not merge logically non-contiguous requests

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Message-ID: <20100521153745.GP32248@dhcp231-156.rdu.redhat.com>
Date:	Fri, 21 May 2010 11:37:45 -0400
From:	Josef Bacik <josef@...hat.com>
To:	Christoph Hellwig <hch@...radead.org>
Cc:	Josef Bacik <josef@...hat.com>, linux-btrfs@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	akpm@...ux-foundation.org
Subject: Re: [PATCH 3/6] direct-io: do not merge logically non-contiguous
	requests

On Fri, May 21, 2010 at 11:21:11AM -0400, Christoph Hellwig wrote:
> On Wed, May 19, 2010 at 04:24:51PM -0400, Josef Bacik wrote:
> > Btrfs cannot handle having logically non-contiguous requests submitted.  For
> > example if you have
> > 
> > Logical:  [0-4095][HOLE][8192-12287]
> > Physical: [0-4095]      [4096-8191]
> > 
> > Normally the DIO code would put these into the same BIO's.  The problem is we
> > need to know exactly what offset is associated with what BIO so we can do our
> > checksumming and unlocking properly, so putting them in the same BIO doesn't
> > work.  So add another check where we submit the current BIO if the physical
> > blocks are not contigous OR the logical blocks are not contiguous.
> 
> This gets us slightly less optimal I/O patters for other filesystems in
> this case.  But it's probably corner case enough to not care and make it
> the default.
> 
> But please make the comment in the comment as verbose as the commit
> message so that people understand why we're doing this when reading the
> code in a few years.
> 

So after I sent this I thought that maybe I could make that test _only_ if we
provide submit_bio, that way it only affects btrfs and not everybody else, would
you prefer I do something like that?  I will make the commit log a bit more
verbose.  Thanks,

Josef
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