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 CC1E8C2BA2B for ; Thu, 9 Apr 2020 14:33:36 +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 94515214D8 for ; Thu, 9 Apr 2020 14:33:36 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="aOe8OjsK" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 94515214D8 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]:50758 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1jMYFL-0002wQ-Qh for qemu-devel@archiver.kernel.org; Thu, 09 Apr 2020 10:33:35 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:40458) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1jMYEZ-0002JA-Df for qemu-devel@nongnu.org; Thu, 09 Apr 2020 10:32:48 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1jMYEV-0007kj-SN for qemu-devel@nongnu.org; Thu, 09 Apr 2020 10:32:45 -0400 Received: from us-smtp-1.mimecast.com ([207.211.31.81]:22930 helo=us-smtp-delivery-1.mimecast.com) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1jMYEV-0007kW-C2 for qemu-devel@nongnu.org; Thu, 09 Apr 2020 10:32:43 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1586442762; 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=ujcT+BIDTMUUBh7B9IQDNxoBUzewwRj/dSNEKYP3xJ8=; b=aOe8OjsK3nVQAqpWyb9Z9zsf7pkNMMz82Wzq8Cjejznmwjz5EdpH7JZUz7nWebdwGg3mu9 ZLJVt+9nGx3aDq5dG8oWqaw/2ASpZ1RoZI+gpGXKI9V1OfZQbQS71BWB0cEnA4m0cTREgt AUbxVa6tJzyraEjHZmmDjSSto7p+HjI= 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-169-HgnFJ4hsPP2n2moqsfepRQ-1; Thu, 09 Apr 2020 10:32:38 -0400 X-MC-Unique: HgnFJ4hsPP2n2moqsfepRQ-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 67C2D1005509; Thu, 9 Apr 2020 14:32:37 +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 E56B79DD89; Thu, 9 Apr 2020 14:32:36 +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> From: Eric Blake Organization: Red Hat, Inc. Message-ID: Date: Thu, 9 Apr 2020 09:32:36 -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: 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] [fuzzy] X-Received-From: 207.211.31.81 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 9:10 AM, Max Reitz wrote: >> >> What happens when an operation attempts to unmap things?=C2=A0 Do we rej= ect >> 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. qcow2_free_any_clusters() passes > them through to the external data file. >=20 > 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 si= de of > things. However, I=E2=80=99m not sure whether we support modifications o= f a raw > file when it is already =E2=80=9Cin use=E2=80=9D by a qcow2 image, so may= be that=E2=80=99s fine. We don't support concurrent modification. But if the guest is running=20 and unmaps things, then shuts off, then we edit the raw file offline,=20 then we restart the guest, the guest should see the results of those=20 offline edits. We have to special-case the qcow2 code to either treat unallocated=20 cluster + raw-data-file as not really unallocated (so that we can see=20 those edits), or we have to special case it to handle unmap +=20 raw-data-file specially (pass unmap to the raw file, but do NOT mark the=20 cluster unallocated in the qcow2 wrapper). Which special case we choose=20 for unmap may in turn affect whether it is easier to require=20 preallocation=3Dmetadata (because we can now guarantee that no cluster=20 will ever be marked unallocated in qcow2) or whether the default=20 preallocation of leaving clusters "unallocated" in qcow2 still sees the=20 proper guest data. --=20 Eric Blake, Principal Software Engineer Red Hat, Inc. +1-919-301-3226 Virtualization: qemu.org | libvirt.org