From mboxrd@z Thu Jan 1 00:00:00 1970 From: Christoph Hellwig Subject: Re: [PATCH +stable] block: don't attempt to merge overlapping requests Date: Wed, 19 May 2010 11:31:01 +0200 Message-ID: <20100519093101.GA6403@lst.de> References: <1274203124-14318-1-git-send-email-avi@redhat.com> <4BF39CA3.9020305@redhat.com> <4BF3AA1E.4050307@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Avi Kivity , Kevin Wolf , qemu-devel@nongnu.org, kvm@vger.kernel.org, Christoph Hellwig To: Stefan Hajnoczi Return-path: Received: from verein.lst.de ([213.95.11.210]:33105 "EHLO verein.lst.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752847Ab0ESJbT (ORCPT ); Wed, 19 May 2010 05:31:19 -0400 Content-Disposition: inline In-Reply-To: Sender: kvm-owner@vger.kernel.org List-ID: On Wed, May 19, 2010 at 10:23:44AM +0100, Stefan Hajnoczi wrote: > On Wed, May 19, 2010 at 10:06 AM, Avi Kivity wrote: > >> In the cache=writeback case the virtio-blk guest driver does: > >> > >> blk_queue_ordered(q, QUEUE_ORDERED_DRAIN_FLUSH, ...) > >> > > > > I don't follow. ?What's the implication? > > I was wondering whether the queue is incorrectly set to a mode where > overlapping write requests aren't being ordered. Anyway, Christoph > says overlapping write requests can be issued so my theory is broken. They can happen, but won't during usual pagecache based writeback. So this should not happen for the pagecache based mke2fs workload. It could happen for a workload with mkfs that uses O_DIRECT. And we need to handle it either way. And btw, our barrier handling for devices using multiwrite (aka virtio) is utterly busted right now, patch will follow ASAP.