linux-kernel - Re: Distributed storage.

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Message-ID: <20070813100636.GL23758@kernel.dk>
Date:	Mon, 13 Aug 2007 12:06:37 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Daniel Phillips <phillips@...nq.net>
Cc:	Evgeniy Polyakov <johnpol@....mipt.ru>, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: Distributed storage.

On Mon, Aug 13 2007, Daniel Phillips wrote:
> On Monday 13 August 2007 02:13, Jens Axboe wrote:
> > On Mon, Aug 13 2007, Daniel Phillips wrote:
> > > On Monday 13 August 2007 00:45, Jens Axboe wrote:
> > > > On Mon, Aug 13 2007, Jens Axboe wrote:
> > > > > > You did not comment on the one about putting the bio
> > > > > > destructor in the ->endio handler, which looks dead simple. 
> > > > > > The majority of cases just use the default endio handler and
> > > > > > the default destructor.  Of the remaining cases, where a
> > > > > > specialized destructor is needed, typically a specialized
> > > > > > endio handler is too, so combining is free.  There are few if
> > > > > > any cases where a new specialized endio handler would need to
> > > > > > be written.
> > > > >
> > > > > We could do that without too much work, I agree.
> > > >
> > > > But that idea fails as well, since reference counts and IO
> > > > completion are two completely seperate entities. So unless end IO
> > > > just happens to be the last user holding a reference to the bio,
> > > > you cannot free it.
> > >
> > > That is not a problem.  When bio_put hits zero it calls ->endio
> > > instead of the destructor.  The ->endio sees that the count is zero
> > > and destroys the bio.
> >
> > You can't be serious? You'd stall end io completion notification
> > because someone holds a reference to a bio.
> 
> Of course not.  Nothing I said stops endio from being called in the 
> usual way as well.  For this to work, endio just needs to know that one 
> call means "end" and the other means "destroy", this is trivial.

Sorry Daniel, but your suggestions would do nothing more than uglify the
code and design.

-- 
Jens Axboe

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