From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([208.118.235.92]:33702) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1UHejE-0003B6-Tl for qemu-devel@nongnu.org; Mon, 18 Mar 2013 14:24:15 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1UHejC-0005Ze-JS for qemu-devel@nongnu.org; Mon, 18 Mar 2013 14:24:12 -0400 Received: from mx1.redhat.com ([209.132.183.28]:63490) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1UHejC-0005ZA-9U for qemu-devel@nongnu.org; Mon, 18 Mar 2013 14:24:10 -0400 Message-ID: <5147562E.1090203@redhat.com> Date: Mon, 18 Mar 2013 19:00:14 +0100 From: Paolo Bonzini MIME-Version: 1.0 References: <1363609123-20748-1-git-send-email-alex@alex.org.uk> <51471767.8030604@redhat.com> <7AC8953FE45335FB794B6DFE@Ximines.local> <51471F14.7030209@redhat.com> <6D0F4ACDA3B7FCF1A50F8B52@Ximines.local> <5147298C.8080900@redhat.com> <51473E82.1020806@redhat.com> <861AFE1A9C44444FD8BAEE16@Ximines.local> <5147512E.5050501@eu.citrix.com> In-Reply-To: <5147512E.5050501@eu.citrix.com> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] [PATCHv3] QEMU(upstream): Disable xen's use of O_DIRECT by default as it results in crashes. List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: George Dunlap Cc: Ian Campbell , Stefano Stabellini , Ian Jackson , "qemu-devel@nongnu.org" , xen-devel , Alex Bligh , Anthony Liguori Il 18/03/2013 18:38, George Dunlap ha scritto: >>> >> This might be a difference between Xen and KVM. On Xen migration is >> made to a server in a paused state, and it's only unpaused when >> the migration to B is complete. There's a sort of extra handshake at >> the end. > > I think what you mean is that all the memory is handled by Xen and the > toolstack, not by qemu. The qemu state is sent as the very last thing, > after all of the memory, and therefore (you are arguing) that qemu is > not started, and the files cannot be opened, until after the migration > is nearly complete, and certainly until after the file is closed on the > sending side. That would be quite dangerous. Files aren't closed until after QEMU exits; at this point whatever problem you have launching QEMU on the destination would be unrecoverable. Even for successful migration, it would also be bad for downtime (QEMU isn't exactly lightning-fast to start). And even if failure weren't catastrophic, it would be a pity to transfer a few gigs of memory and then find out that QEMU isn't present in the destination. :) Still, it's more than possible that I've forgotten something about Xen's management of QEMU. > (In KVM this isn't the case because I assume it's qemu that handles the > memory transition, and so it is likely to open the files on the > receiving side when the migration starts.) Yup. Paolo