From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from [140.186.70.92] (port=41658 helo=eggs.gnu.org) by lists.gnu.org with esmtp (Exim 4.43) id 1OGvcn-0001in-4G for qemu-devel@nongnu.org; Tue, 25 May 2010 11:00:59 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.69) (envelope-from ) id 1OGvce-0004RB-KU for qemu-devel@nongnu.org; Tue, 25 May 2010 11:00:56 -0400 Received: from mx1.redhat.com ([209.132.183.28]:8948) by eggs.gnu.org with esmtp (Exim 4.69) (envelope-from ) id 1OGvce-0004Qs-94 for qemu-devel@nongnu.org; Tue, 25 May 2010 11:00:48 -0400 Message-ID: <4BFBE614.9080306@redhat.com> Date: Tue, 25 May 2010 18:00:36 +0300 From: Avi Kivity MIME-Version: 1.0 Subject: Re: [Qemu-devel] [RFC PATCH 1/1] ceph/rbd block driver for qemu-kvm References: <20100519192222.GD61706@ncolin.muc.de> <4BF5A9D2.5080609@codemonkey.ws> <4BF91937.2070801@redhat.com> <4BFBAE46.5050801@redhat.com> <4BFBB3C1.9020905@redhat.com> <4BFBCFAC.9070807@codemonkey.ws> <4BFBD13C.60605@redhat.com> <4BFBD20E.5060207@codemonkey.ws> <4BFBD2D5.2000201@redhat.com> <4BFBD6CD.3000503@codemonkey.ws> <4BFBD82F.3020404@redhat.com> <4BFBD943.8020704@codemonkey.ws> In-Reply-To: <4BFBD943.8020704@codemonkey.ws> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit List-Id: qemu-devel.nongnu.org List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Anthony Liguori Cc: Kevin Wolf , kvm@vger.kernel.org, qemu-devel@nongnu.org, Blue Swirl , ceph-devel@vger.kernel.org, Christian Brunner On 05/25/2010 05:05 PM, Anthony Liguori wrote: > On 05/25/2010 09:01 AM, Avi Kivity wrote: >> On 05/25/2010 04:55 PM, Anthony Liguori wrote: >>> On 05/25/2010 08:38 AM, Avi Kivity wrote: >>>> On 05/25/2010 04:35 PM, Anthony Liguori wrote: >>>>> On 05/25/2010 08:31 AM, Avi Kivity wrote: >>>>>>> A protocol based mechanism has the advantage of being more >>>>>>> robust in the face of poorly written block backends so if it's >>>>>>> possible to make it perform as well as a plugin, it's a >>>>>>> preferable approach. >>>>>> >>>>>> May be hard due to difficulty of exposing guest memory. >>>>> >>>>> If someone did a series to add plugins, I would expect a very >>>>> strong argument as to why a shared memory mechanism was not >>>>> possible or at least plausible. >>>>> >>>>> I'm not sure I understand why shared memory is such a bad thing >>>>> wrt KVM. Can you elaborate? Is it simply a matter of fork()? >>>> >>>> fork() doesn't work in the with of memory hotplug. What else is >>>> there? >>>> >>> >>> Is it that fork() doesn't work or is it that fork() is very expensive? >> >> It doesn't work, fork() is done at block device creation time, which >> freezes the child memory map, while guest memory is allocated at >> hotplug time. > > Now I'm confused. I thought you were saying shared memory somehow > affects fork(). If you're talking about shared memory inheritance via > fork(), that's less important. The latter. Why is it less important? If you don't inherit the memory, you can't access it. > You can also pass /dev/shm fd's via SCM_RIGHTs to establish shared > memory segments dynamically. Doesn't work for anonymous memory. -- error compiling committee.c: too many arguments to function