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 3324DC2BA2B for ; Thu, 9 Apr 2020 13:44:04 +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 EB10E208FE for ; Thu, 9 Apr 2020 13:44:03 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="dH6EDT66" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org EB10E208FE 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]:49540 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1jMXTP-0004Mp-46 for qemu-devel@archiver.kernel.org; Thu, 09 Apr 2020 09:44:03 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:60258) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1jMXSj-0003vA-NE for qemu-devel@nongnu.org; Thu, 09 Apr 2020 09:43:22 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1jMXSi-00008m-KK for qemu-devel@nongnu.org; Thu, 09 Apr 2020 09:43:21 -0400 Received: from us-smtp-delivery-1.mimecast.com ([207.211.31.120]:36405 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 1jMXSi-00008X-HY for qemu-devel@nongnu.org; Thu, 09 Apr 2020 09:43:20 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1586439800; 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=tqpqM4zPlmC8+9rzbrXkEdDbEIOx4neVz1jbG3y3kBI=; b=dH6EDT66RcIMkFW0qxgO2H4PDCVLyQfY0tD/w02BgslPibX9rRDet47ji24psQ6dDdQc/S AApm5ASvVLh+xti1rjAP//KuM2eTGFg1u8nX+O4lvWx63LhGxstHfcjsunbghuzN9hUt+m tOTMVUu2PdPBQ0iR8XxJFW462pyIapY= 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-475-vMM5dpe-NuW8IW4zHEaR_A-1; Thu, 09 Apr 2020 09:43:04 -0400 X-MC-Unique: vMM5dpe-NuW8IW4zHEaR_A-1 Received: from smtp.corp.redhat.com (int-mx04.intmail.prod.int.phx2.redhat.com [10.5.11.14]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 75042801E6C; Thu, 9 Apr 2020 13:43:03 +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 A8F705E242; Thu, 9 Apr 2020 13:42:56 +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 08:42:55 -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: <8b4bc264-7bce-c9c1-1905-a22b4c61cae4@redhat.com> Content-Language: en-US X-Scanned-By: MIMEDefang 2.79 on 10.5.11.14 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: 207.211.31.120 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 8:05 AM, Max Reitz wrote: >>> $ sudo ./qemu-img create -f qcow2 -o \ >>> =C2=A0=C2=A0=C2=A0=C2=A0 data_file=3D/dev/loop0,data_file_raw=3Don foo= .qcow2 64M >>> [...] >>> >>> $ sudo ./qemu-io -c 'read -P 0 0 64M' foo.qcow2 >>> read 67108864/67108864 bytes at offset 0 >>> 64 MiB, 1 ops; 00.00 sec (25.036 GiB/sec and 400.5751 ops/sec) >> >> This looks like a bug (and we should fix it for 5.0 if possible) >=20 > It seems a bit difficult for 5.0 now. (But I don=E2=80=99t think it=E2= =80=99d be a > regression, so that shouldn=E2=80=99t be too bad.) So, you're arguing that since 4.2 has the same bug, slipping the fix to=20 5.1 instead of 5.0 is not bad because it's not a regression new to 5.0.=20 Yes, that's a reasonable answer, if a fix is not fast. >>> I suppose this behavior is fine for blockdev-create because I guess it= =E2=80=99s >>> the user=E2=80=99s responsibility to ensure that the external data file= is zero. >>> =C2=A0 But maybe it isn=E2=80=99t, so that=E2=80=99s my first question= : Is it really the >>> user=E2=80=99s responsibility or should we always ensure it=E2=80=99s z= ero? >> >> I'd argue that requiring the user to pre-zero the raw data file is >> undesirable; and that we should instead fix our code to not report the >> image as reading all zeroes when creating with data_file_raw=3Don. >=20 > OK. I think that could be achieved by just enforcing @preallocation to > be at least =E2=80=9Cmetadata=E2=80=9D whenever @data-file-raw is set. W= ould that make > sense? Is a preallocation of metadata sufficient to report things correctly?=20 If so, it seems like a reasonable compromise to me. I was more=20 envisioning a fix elsewhere: if we are reporting block status of what=20 looks like an unallocated cluster, but data-file-raw is set, we change=20 our answer to instead report it as allocated with unknown contents. But=20 with preallocation, you either force the qcow2 file to list no cluster=20 as unallocated (which matches the fact that the raw image really is=20 fully allocated) while not touching the raw image, or you can go one=20 step further and request full preallocation to wipe the raw image to 0=20 in the process. >=20 > Max >=20 >>> My second question is: If we decide that this is fine for >>> blockdev-create, should at least qcow2_co_create_opts() ensure the data >>> file it just created is zero? >> >> Having an option to make qemu force-zero the raw image during >> qcow2_co_create_opts seems reasonable, but for performance reasons, I >> don't think the flag should be on by default. And by mentioning preallocation, you've managed to convince me that we=20 may already have exactly the option I was envisioning. --=20 Eric Blake, Principal Software Engineer Red Hat, Inc. +1-919-301-3226 Virtualization: qemu.org | libvirt.org