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Date: Thu, 10 Jan 2002 11:51:51 +0100
From: Jens Axboe <axboe@suse.de>
To: Andrea Arcangeli <andrea@suse.de>
Cc: Badari Pulavarty <pbadari@us.ibm.com>, Benjamin LaHaise <bcrl@redhat.com>,
        linux-kernel@vger.kernel.org, marcelo@conectiva.com.br
Subject: Re: [PATCH] PAGE_SIZE IO for RAW (RAW VARY)
Message-ID: <20020110115151.T19814@suse.de>
In-Reply-To: <20020109132148.C12609@redhat.com> <200201091928.g09JSdH23535@eng2.beaverton.ibm.com> <20020110111825.C3357@inspiron.school.suse.de> <20020110112225.S19814@suse.de> <20020110114704.I3357@inspiron.school.suse.de>
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On Thu, Jan 10 2002, Andrea Arcangeli wrote:
> On Thu, Jan 10, 2002 at 11:22:25AM +0100, Jens Axboe wrote:
> > On Thu, Jan 10 2002, Andrea Arcangeli wrote:
> > > On Wed, Jan 09, 2002 at 11:28:39AM -0800, Badari Pulavarty wrote:
> > > > Ben,
> > > > 
> > > > By any chance do you have a list of drivers that assume this ?
> > > > What does it take to fix them ? 
> > > > 
> > > > I think Jens BIO changes for 2.5 will fix this problem. But 2.4
> > > > needs a solution in this area too. This patch showed significant
> > > > improvement for database workloads. 
> > > 
> > > I didn't checked the implementation but as far as the blkdev is
> > > concerned the b_size changes without notification as soon as you
> > > 'mkfs -b somethingelse' and then mount the fs. So it cannot break
> > > as far I can tell. The only important thing is that b_size stays
> > > between 512 and 4k.
> > 
> > The concern is/was differently sized buffer_heads in the same
> > request, ie b_size changing as you iterate through the chunks of one
> > request.
> 
> ok, I don't expect problems there. It can happen for example if you
> create a snapshot with 4k and then you switch back the original volume
> to 1k. the physical volume will get mixed b_size colaesced into the
> same request.

Well I don't expect problems either, however Jeff Merkey did report them
but see my previous mail on that (validity of that report is
questionable).

I still wouldn't feel to good doing this, and just because snapshotting
opens the possibility for this to happen doesn't mean it a) ever
triggered in real life, and b) works on all devices.

-- 
Jens Axboe

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