linux-kernel - Re: Linux 2.6.29

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Date:	Thu, 26 Mar 2009 19:32:54 +0100
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Hugh Dickins <hugh@...itas.com>
Cc:	Ric Wheeler <rwheeler@...hat.com>, Jeff Garzik <jeff@...zik.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Theodore Tso <tytso@....edu>, Ingo Molnar <mingo@...e.hu>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Arjan van de Ven <arjan@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Nick Piggin <npiggin@...e.de>, David Rees <drees76@...il.com>,
	Jesper Krogh <jesper@...gh.cc>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Linux 2.6.29

On Thu, Mar 26 2009, Hugh Dickins wrote:
> On Thu, 26 Mar 2009, Jens Axboe wrote:
> > On Thu, Mar 26 2009, Hugh Dickins wrote:
> > > On Thu, 26 Mar 2009, Jens Axboe wrote:
> > > > On Wed, Mar 25 2009, Hugh Dickins wrote:
> > > > > 
> > > > > Tangential question, but am I right in thinking that BIO_RW_BARRIER
> > > > > similarly bars across all partitions, whereas its WRITE_BARRIER and
> > > > > DISCARD_BARRIER users would actually prefer it to apply to just one?
> > > > 
> > > > All the barriers refer to just that range which the barrier itself
> > > > references.
> > > 
> > > Ah, thank you: then I had a fundamental misunderstanding of them,
> > > and need to go away and work that out some more.
> > > 
> > > Though I didn't read it before asking, doesn't the I/O Barriers section
> > > of Documentation/block/biodoc.txt give a very different impression?
> > 
> > I'm sensing a miscommunication here... The ordering constraint is across
> > devices, at least that is how it is implemented. For file system
> > barriers (like BIO_RW_BARRIER), it could be per-partition instead. Doing
> > so would involve some changes at the block layer side, not necessarily
> > trivial. So I think you were asking about ordering, I was answering
> > about the write guarantee :-)
> 
> Ah, thank you again, perhaps I did understand after all.
> 
> So, directing a barrier (WRITE_BARRIER or DISCARD_BARRIER) to a range
> of sectors in one partition interposes a barrier into the queue of I/O
> across (all partitions of) that whole device.

Correct

> I think that's not how filesystems really want barriers to behave,
> and might tend to discourage us from using barriers more freely.
> But I have zero appreciation of whether it's a significant issue
> worth non-trivial change - just wanted to get it out into the open.

Per-partition definitely makes sense. The problem is that we do sorting
on a per-device basis right now. But it's a good point, I'll try and
take a look at how much work it would be to make it per-partition
instead. It wont be trivial :-)

-- 
Jens Axboe

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