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=-2.4 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_SANE_1 autolearn=no 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 D7E4CC2BA2B for ; Thu, 9 Apr 2020 15:48:23 +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 9FA0E20769 for ; Thu, 9 Apr 2020 15:48:23 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="Xqir8LVi" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 9FA0E20769 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]:52014 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1jMZPi-0000Mt-RE for qemu-devel@archiver.kernel.org; Thu, 09 Apr 2020 11:48:22 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:52466) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1jMZOW-0007uS-8E for qemu-devel@nongnu.org; Thu, 09 Apr 2020 11:47:09 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1jMZOU-0003nG-TQ for qemu-devel@nongnu.org; Thu, 09 Apr 2020 11:47:07 -0400 Received: from us-smtp-delivery-1.mimecast.com ([205.139.110.120]:38887 helo=us-smtp-1.mimecast.com) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1jMZOU-0003ju-QP for qemu-devel@nongnu.org; Thu, 09 Apr 2020 11:47:06 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1586447222; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=0SnoqIWJLdqz7zO7Umi3yzoNXL2HaHL7mANxY0tVX3g=; b=Xqir8LVimHfIWyMWGXAyxCU+FRtUVEg0mlmY3a5JJ6HoaTNSeN8cbPX94WGcq0bq9Wwab3 HgYr8ByAWfbRK53kou6sZdfxjDMwR1o9iTMrhezjbTXY59vSGiDffrnvB49ftX7jaZb2gK jp58A67nbZVxa6yr7SqQanWVxFg2xSQ= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-64-eoFD45Q8MiGaVf8zLzBeew-1; Thu, 09 Apr 2020 11:46:59 -0400 X-MC-Unique: eoFD45Q8MiGaVf8zLzBeew-1 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 mimecast-mx01.redhat.com (Postfix) with ESMTPS id 172CD8017FC; Thu, 9 Apr 2020 15:46:58 +0000 (UTC) Received: from [10.3.114.49] (ovpn-114-49.phx2.redhat.com [10.3.114.49]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 93E6352735; Thu, 9 Apr 2020 15:46:57 +0000 (UTC) Subject: Re: qcow2: Zero-initialization of external data files To: Max Reitz , Qemu-block , Kevin Wolf References: <50080252-ff22-78ed-0002-1742c694471b@redhat.com> <8b4bc264-7bce-c9c1-1905-a22b4c61cae4@redhat.com> <713d39ff-29f6-f9e0-bbbc-c9b26ffd28a0@redhat.com> From: Eric Blake Organization: Red Hat, Inc. Message-ID: <5a8099d6-3885-2bfe-f85a-477c5cc76a45@redhat.com> Date: Thu, 9 Apr 2020 10:46:56 -0500 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.6.0 MIME-Version: 1.0 In-Reply-To: <713d39ff-29f6-f9e0-bbbc-c9b26ffd28a0@redhat.com> Content-Language: en-US X-Scanned-By: MIMEDefang 2.79 on 10.5.11.13 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: quoted-printable X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] X-Received-From: 205.139.110.120 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: "qemu-devel@nongnu.org" Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 4/9/20 10:01 AM, Max Reitz wrote: > On 09.04.20 16:32, Eric Blake wrote: >> On 4/9/20 9:10 AM, Max Reitz wrote: >> >>>> >>>> What happens when an operation attempts to unmap things?=C2=A0 Do we r= eject >>>> all unmap operations when data-file-raw is set (thus leaving a cluster >>>> marked as allocated at all times, if we can first guarantee that >>>> preallocation set things up that way)? >>> No, unmap operations currently work.=C2=A0 qcow2_free_any_clusters() pa= sses >>> them through to the external data file. >>> >>> The problem is that the unmap also zeroes the L2 entry, so if you then >>> write data to the raw file, it won=E2=80=99t be visible from the qcow2 = side of >>> things.=C2=A0 However, I=E2=80=99m not sure whether we support modifica= tions of a raw >>> file when it is already =E2=80=9Cin use=E2=80=9D by a qcow2 image, so m= aybe that=E2=80=99s fine. >> >> We don't support concurrent modification. But if the guest is running >> and unmaps things, then shuts off, then we edit the raw file offline, >> then we restart the guest, the guest should see the results of those >> offline edits. >=20 > Should it? The specification doesn=E2=80=99t say anything about that. >=20 > In fact, I think we have always said we explicitly discourage that > because this might lead to outdated metadata; even though we usually > meant =E2=80=9Cdirty bitmaps=E2=80=9D by that. Hmm. Kevin, I'd really like your opinion here. The point of the=20 raw-external-data flag is to state that "qemu MUST ensure that whatever=20 is done to this image while the guest is running is reflected through to=20 the raw file, so that after the guest stops, the raw file alone is still=20 viable to see what the guest saw". But as you say, there's a difference=20 between "the raw file will read what the guest saw" and "we can now edit=20 the raw file without regards to the qcow2 wrapper but later reuse of the=20 qcow2 wrapper won't be corrupted by those edits". --=20 Eric Blake, Principal Software Engineer Red Hat, Inc. +1-919-301-3226 Virtualization: qemu.org | libvirt.org