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Date: Wed, 29 Jun 2011 06:01:16 -0400
From: Christoph Hellwig <hch@infradead.org>
Message-ID: <20110629100116.GB22900@infradead.org>
References: <4DEE2B15.4090809@redhat.com> <4DF21447.6090005@suse.de>
	<20110612075140.GB11941@redhat.com>
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Subject: Re: [Qemu-devel] virtio scsi host draft specification, v3
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To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Christoph Hellwig <chellwig@redhat.com>, Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>, "kvm@vger.kernel.org" <kvm@vger.kernel.org>, Rusty Russell <rusty@rustcorp.com.au>, qemu-devel <qemu-devel@nongnu.org>, Linux Kernel Mailing List <linux-kernel@vger.kernel.org>, Hannes Reinecke <hare@suse.de>, Paolo Bonzini <pbonzini@redhat.com>, Linux Virtualization <virtualization@lists.linux-foundation.org>

On Sun, Jun 12, 2011 at 10:51:41AM +0300, Michael S. Tsirkin wrote:
> For example, if the driver is crazy enough to put
> all write requests on one queue and all barriers
> on another one, how is the device supposed to ensure
> ordering?

There is no such things as barriers in SCSI.  The thing that comes
closest is ordered tags, which neither Linux nor any mainstream OS
uses, and which we don't have to (and generally don't want to)
implement.