From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-5.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 50472C47404 for ; Fri, 11 Oct 2019 09:02:42 +0000 (UTC) Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id 229BE2084C for ; Fri, 11 Oct 2019 09:02:42 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 229BE2084C Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Received: from localhost ([::1]:47498 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1iIqoZ-0001JY-Qk for qemu-devel@archiver.kernel.org; Fri, 11 Oct 2019 05:02:23 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:48403) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1iIqmw-000080-BX for qemu-devel@nongnu.org; Fri, 11 Oct 2019 05:00:43 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1iIqmu-0001B8-U8 for qemu-devel@nongnu.org; Fri, 11 Oct 2019 05:00:42 -0400 Received: from mx1.redhat.com ([209.132.183.28]:49872) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1iIqmu-0001AE-LH for qemu-devel@nongnu.org; Fri, 11 Oct 2019 05:00:40 -0400 Received: from smtp.corp.redhat.com (int-mx03.intmail.prod.int.phx2.redhat.com [10.5.11.13]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 81ECA30A7B81; Fri, 11 Oct 2019 09:00:39 +0000 (UTC) Received: from localhost.localdomain (unknown [10.36.118.111]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 7319A60872; Fri, 11 Oct 2019 09:00:38 +0000 (UTC) Date: Fri, 11 Oct 2019 11:00:36 +0200 From: Kevin Wolf To: Markus Armbruster Subject: Re: [PATCH v2 2/2] qapi: Allow introspecting fix for savevm's cooperation with blockdev Message-ID: <20191011090036.GA5158@localhost.localdomain> References: <992ea9ca130b4fb6dbf82726aa3b1d8040c16944.1568989362.git.pkrempa@redhat.com> <87r23w2rbp.fsf@dusky.pond.sub.org> <87a7ajuzpw.fsf@dusky.pond.sub.org> <20191010150733.GE7616@localhost.localdomain> <87r23jlsql.fsf@dusky.pond.sub.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <87r23jlsql.fsf@dusky.pond.sub.org> User-Agent: Mutt/1.12.1 (2019-06-15) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.13 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.47]); Fri, 11 Oct 2019 09:00:39 +0000 (UTC) X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] [fuzzy] X-Received-From: 209.132.183.28 X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Peter Krempa , qemu-devel@nongnu.org, Michael Roth Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" Am 11.10.2019 um 08:08 hat Markus Armbruster geschrieben: > Kevin Wolf writes: > > > Am 02.10.2019 um 13:57 hat Markus Armbruster geschrieben: > >> Eric Blake writes: > >> > >> > On 10/1/19 2:34 PM, Markus Armbruster wrote: > >> >> Peter Krempa writes: > >> >> > >> >>> savevm was buggy as it considered all monitor owned block device nodes > >> >> > >> >> Recommend "monitor-owned block device nodes" or "block device nodes > >> >> owned by a monitor" > >> >> > >> >>> for snapshot. With introduction of -blockdev the common usage made all > >> >>> nodes including protocol nodes monitor owned and thus considered for > >> >>> snapshot. > >> >> > >> >> What exactly is / was the problem? > >> > > >> > > >> > Old way: using QMP add_device, you create a drive backend with two BDS > >> > (format and protocol) assigned to it; the drive backend has your given > >> > name, and both BDS have a generated name (beginning with '#'). The > >> > two BDS are not monitor-owned, rather, the drive is. > >> > > >> > New way: using QMP blockdev_add, you create the two BDS manually with > >> > names of your choice, then plug that blockdev into an unnamed > >> > blockbackend (the drive no longer needs a name, because you can get at > >> > everything through the BDS name). You _could_ do this in one step > >> > (the QAPI allows self-recursion where you can define both the format > >> > and protocol in one step), but it is easier to do in two steps (define > >> > the protocol BDS first, then define the format BDS using a "string" > >> > name of the protocol BDS instead of a { "driver":..., args... } object > >> > of the protocol layer. But by making two calls, now both BDS are > >> > monitor-owned. > >> > > >> > At snapshot-time, the code currently looks for all monitor-owned nodes > >> > when deciding what to snapshot. In the old way, this finds the named > >> > drive, picks up its associated top-most node, and snapshots the format > >> > layer. In the new way, the drive is unnamed so it is skipped, while > >> > there are two named BDS, but we don't want a snapshot of the protocol > >> > layer. > >> > >> So the problem is certain (common & sane) -blockdev use makes savevm > >> create additional, unwanted snapshots. > > > > Actually, the most common protocol driver is file-posix, which doesn't > > support snapshots, so usually the result was that savevm just fails > > because it can't snapshot something that it (incorrectly) thinks it > > should snapshot. > > v3's commit message: > > qapi: Allow introspecting fix for savevm's cooperation with blockdev > > 'savevm' was buggy as it considered all monitor-owned block device nodes > for snapshot. With introduction of -blockdev the common usage made all > nodes including protocol and backing file nodes monitor-owned and thus > considered for snapshot. > > This is a problem since the 'file' protocol nodes can't have internal > snapshots and it does not make sense to take snapshot of nodes > representing backing files. > > This was fixed by commit 05f4aced658a02b02 clients need to be able to > detect whether this fix is present. Something is missing in this sentence. I think you lost the "but" from the original message. > Since savevm does not have an QMP alternative, add the feature for the > 'human-monitor-command' backdoor which is used to call this command in > modern use. > > Signed-off-by: Peter Krempa > > Kevin, is this explanation sufficiently correct & complete? Looks good to me otherwise. Kevin