From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mailman by lists.gnu.org with tmda-scanned (Exim 4.43) id 1MPzu9-0006aG-Ew for qemu-devel@nongnu.org; Sun, 12 Jul 2009 10:19:49 -0400 Received: from exim by lists.gnu.org with spam-scanned (Exim 4.43) id 1MPzu5-0006Wh-12 for qemu-devel@nongnu.org; Sun, 12 Jul 2009 10:19:49 -0400 Received: from [199.232.76.173] (port=53580 helo=monty-python.gnu.org) by lists.gnu.org with esmtp (Exim 4.43) id 1MPzu4-0006We-Qs for qemu-devel@nongnu.org; Sun, 12 Jul 2009 10:19:44 -0400 Received: from mx2.redhat.com ([66.187.237.31]:38406) by monty-python.gnu.org with esmtp (Exim 4.60) (envelope-from ) id 1MPzu4-0004gV-DA for qemu-devel@nongnu.org; Sun, 12 Jul 2009 10:19:44 -0400 Message-ID: <4A59F1AC.9070401@redhat.com> Date: Sun, 12 Jul 2009 17:22:36 +0300 From: Avi Kivity MIME-Version: 1.0 Subject: Re: [Qemu-devel] [PATCH 2/3] move vm stop/start to migrate_set_state References: <1247140059-5034-1-git-send-email-pbonzini@redhat.com> <1247140059-5034-3-git-send-email-pbonzini@redhat.com> <4A55F46F.6060705@codemonkey.ws> <4A55F510.5090801@redhat.com> <4A55F641.6000701@codemonkey.ws> <20090710231424.GD30322@shareable.org> <4A57E3AA.5020305@codemonkey.ws> <20090711014207.GM30322@shareable.org> <4A5958F8.3090306@codemonkey.ws> In-Reply-To: <4A5958F8.3090306@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: Paolo Bonzini , qemu-devel@nongnu.org On 07/12/2009 06:31 AM, Anthony Liguori wrote: > Jamie Lokier wrote: >> If you get an error during the last write(), I wouldn't trust that to >> mean the recipient will definitely not see the data you wrote. (Enjoy >> the double negative). It's another variation of the handshake >> uncertainty, this time reflected in what write() should report when >> it's uncertain about a network transmission. If it reports an error >> when it's uncertain, then you can't trust that a write() error means >> the data was not written, only that a problem was detected. > > I think you're stretching here. If it really were the case that > write() could actually result in data being sent out the wire and yet > still returning an error, it would make all error handling in Unix > unmanagable. I can't believe this is possible in Linux and without an > actual counter-example, I'm inclined to believe the same is true for > every other OS out there. It's actually a common scenario for block devices. I don't know about networking, but for disks a write can be completed and then report an error if the cable or power was disconnected before the acknowledge could arrive. It could conceivably happen with networking if the device reports an error when it isn't sure if the data was sent out or not (but it actually was), or if some path after the transmission required a memory allocation, which failed. -- error compiling committee.c: too many arguments to function