Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932070AbZCZSXa (ORCPT ); Thu, 26 Mar 2009 14:23:30 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1755193AbZCZSXU (ORCPT ); Thu, 26 Mar 2009 14:23:20 -0400 Received: from extu-mxob-1.symantec.com ([216.10.194.28]:48379 "EHLO extu-mxob-1.symantec.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754414AbZCZSXU (ORCPT ); Thu, 26 Mar 2009 14:23:20 -0400 Date: Thu, 26 Mar 2009 18:21:27 +0000 (GMT) From: Hugh Dickins X-X-Sender: hugh@blonde.anvils To: Jens Axboe cc: Ric Wheeler , Jeff Garzik , Linus Torvalds , Theodore Tso , Ingo Molnar , Alan Cox , Arjan van de Ven , Andrew Morton , Peter Zijlstra , Nick Piggin , David Rees , Jesper Krogh , Linux Kernel Mailing List Subject: Re: Linux 2.6.29 In-Reply-To: <20090326154610.GH27476@kernel.dk> Message-ID: References: <49C93AB0.6070300@garzik.org> <20090325093913.GJ27476@kernel.dk> <49CA86BD.6060205@garzik.org> <20090325194341.GB27476@kernel.dk> <49CA8ADA.3040709@redhat.com> <20090325195747.GC27476@kernel.dk> <20090326085748.GH27476@kernel.dk> <20090326154610.GH27476@kernel.dk> MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1947 Lines: 42 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. 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. Hugh -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/