From mboxrd@z Thu Jan 1 00:00:00 1970 From: Anthony Liguori Subject: Re: [PATCH] qemu: qemu_fopen_fd: differentiate between reader and writer user Date: Thu, 16 Oct 2008 21:47:35 -0500 Message-ID: <48F7FCC7.2020108@codemonkey.ws> References: <1223829030-14962-1-git-send-email-uril@qumranet.com> <48F22BF1.3000608@redhat.com> <48F23D4D.2050709@codemonkey.ws> <48F23F42.10405@redhat.com> <48F277A0.8040407@codemonkey.ws> <48F2BA83.7000101@codemonkey.ws> <48F69AAB.4010404@il.qumranet.com> <48F6BFA1.9070608@codemonkey.ws> <48F6F7AA.2080102@redhat.com> <48F7399B.7000808@codemonkey.ws> <48F74E6C.8070100@il.qumranet.com> <48F75078.5090604@redhat.com> <48F75483.1020901@il.qumranet.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: Avi Kivity , kvm@vger.kernel.org To: Uri Lublin Return-path: Received: from yx-out-2324.google.com ([74.125.44.30]:32439 "EHLO yx-out-2324.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752105AbYJQCrl (ORCPT ); Thu, 16 Oct 2008 22:47:41 -0400 Received: by yx-out-2324.google.com with SMTP id 8so77984yxm.1 for ; Thu, 16 Oct 2008 19:47:40 -0700 (PDT) In-Reply-To: <48F75483.1020901@il.qumranet.com> Sender: kvm-owner@vger.kernel.org List-ID: Uri Lublin wrote: > > That is true, but in the case I mentioned above it would take the > management tool some time (guest down time) to realize what happens, > and to send "cont" to the SRC. With end-of-migration messages SRC > discovers DST fails and immediately continues. > I agree those messages add some complexity, and slow things a bit for > the good/average case. It's the classic general's dilemma. If SRC waits for DST to send an ACK, DST still doesn't know whether SRC received the ACK so it doesn't know whether it's truly safe to continue. This is why migration doesn't quit SRC immediately, and leaves SRC in the stopped state. It's because the only safe way to handle this is with a third party that is reliable. Regards, Anthony Liguori