From mboxrd@z Thu Jan 1 00:00:00 1970 From: Avi Kivity Subject: Re: qcow and updates Date: Sat, 26 Jul 2008 12:18:05 +0300 Message-ID: <488AEBCD.3030106@qumranet.com> References: <488A0835.2020202@tmr.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: kvm@vger.kernel.org To: Bill Davidsen Return-path: Received: from il.qumranet.com ([212.179.150.194]:50824 "EHLO il.qumranet.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751453AbYGZJSG (ORCPT ); Sat, 26 Jul 2008 05:18:06 -0400 In-Reply-To: <488A0835.2020202@tmr.com> Sender: kvm-owner@vger.kernel.org List-ID: Bill Davidsen wrote: > This question came out of a discussion of nesting (or stringing if you > like) disk images using qcow. > > If I have an unpatched install image, say CentOS-5.2 (box "A" below). > and I make a copy of that with qcow and apply patches to that copy > (box "B" below), everything is just fine. But if I should make a > working qcow image from the patched version (box "C" below), which > works fine initially, what would happen if I applied patches to the > "current" version? I assume that would mess up the application > version, since it's a physical copy, but I'd like to be sure. > > ______________ > | [A] | > | orig release | > |______________| > || > {qcow copy| > || > ____________________ > | [B] | > | patched to current | > |____________________| > || > {qcow copy} > || > _________________________ > | [C] | > | fully config. app. svr. | > |_________________________| > > I could play with union filesystem, network mounting of /var and /usr, > and other tricks, but I thought I'd check that this really is a > problem, and the current version needs a "real" copy to the app > server, and then the app server needs to be patched after that. > Changing anything except the last image in the chain is liable to break. In other words: once you use an image as a base for another image, this first image must never change. -- Do not meddle in the internals of kernels, for they are subtle and quick to panic.