From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mailman by lists.gnu.org with tmda-scanned (Exim 4.43) id 1LrwWL-0006yi-9Z for qemu-devel@nongnu.org; Thu, 09 Apr 2009 11:50:29 -0400 Received: from exim by lists.gnu.org with spam-scanned (Exim 4.43) id 1LrwWG-0006rI-Bf for qemu-devel@nongnu.org; Thu, 09 Apr 2009 11:50:28 -0400 Received: from [199.232.76.173] (port=55846 helo=monty-python.gnu.org) by lists.gnu.org with esmtp (Exim 4.43) id 1LrwWG-0006r5-6x for qemu-devel@nongnu.org; Thu, 09 Apr 2009 11:50:24 -0400 Received: from mx2.redhat.com ([66.187.237.31]:44875) by monty-python.gnu.org with esmtp (Exim 4.60) (envelope-from ) id 1LrwWF-0002rW-Jq for qemu-devel@nongnu.org; Thu, 09 Apr 2009 11:50:23 -0400 Received: from int-mx2.corp.redhat.com (int-mx2.corp.redhat.com [172.16.27.26]) by mx2.redhat.com (8.13.8/8.13.8) with ESMTP id n39FoMer020263 for ; Thu, 9 Apr 2009 11:50:22 -0400 Received: from ns3.rdu.redhat.com (ns3.rdu.redhat.com [10.11.255.199]) by int-mx2.corp.redhat.com (8.13.1/8.13.1) with ESMTP id n39FoOiM008786 for ; Thu, 9 Apr 2009 11:50:24 -0400 Received: from zweiblum.home.kraxel.org (vpn-10-195.str.redhat.com [10.32.10.195]) by ns3.rdu.redhat.com (8.13.8/8.13.8) with ESMTP id n39FoKtT001355 for ; Thu, 9 Apr 2009 11:50:22 -0400 Message-ID: <49DE193B.50703@redhat.com> Date: Thu, 09 Apr 2009 17:50:19 +0200 From: Gerd Hoffmann MIME-Version: 1.0 Subject: Re: [Qemu-devel] buffer alignment for block backends References: <49DCA80A.1020808@eu.citrix.com> <49DCF2C0.9070704@codemonkey.ws> <49DDC66F.2000404@eu.citrix.com> <49DDF3FF.8050706@codemonkey.ws> <49DDF877.8090805@eu.citrix.com> <49DDFE1A.9040202@redhat.com> <20090409152107.GE5457@const.bordeaux.inria.fr> In-Reply-To: <20090409152107.GE5457@const.bordeaux.inria.fr> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: quoted-printable Reply-To: qemu-devel@nongnu.org List-Id: qemu-devel.nongnu.org List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: qemu-devel@nongnu.org On 04/09/09 17:21, Samuel Thibault wrote: > Gerd Hoffmann, le Thu 09 Apr 2009 15:54:34 +0200, a =E9crit : >> On 04/09/09 15:30, Stefano Stabellini wrote: >>> The userspace\kernelspace distinction does not make much sense in >>> minios, but yes it provides an interface to communicate with blkback = in >>> dom0. This interface is used by block-vbd in qemu instead of >>> block-raw-posix when run in minios. >> Sit back, look again. Do you really need it being page-aligned? > > Page aligned is an optimization which comes for free when you can > already choose the alignment. Ok, so you don't have to. Does it have an effect positive actually? i.e. do you have requests=20 which would fit into one page but actually span two due to the misalignme= nt? At least when watching the linux kernel access pattern I see larger=20 requests being page aligned anyway. Probably page cache reads and writes. >> The xen block protocol can handle unaligned requests just fine. > > Not completely arbitrarily aligned requests. They need to be > sector-aligned. Agreed. Sector-alignment you have anyway, thats why I didn't wrote that=20 explicitly. cheers, Gerd