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,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 47E26C33CB1 for ; Thu, 16 Jan 2020 14:58:42 +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 1116E20748 for ; Thu, 16 Jan 2020 14:58:42 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="Dfc9ku3x" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 1116E20748 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]:43480 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1is6bZ-0007Et-84 for qemu-devel@archiver.kernel.org; Thu, 16 Jan 2020 09:58:41 -0500 Received: from eggs.gnu.org ([2001:470:142:3::10]:57226) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1is6au-0006Q2-4X for qemu-devel@nongnu.org; Thu, 16 Jan 2020 09:58:03 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1is6aq-0007yR-Jc for qemu-devel@nongnu.org; Thu, 16 Jan 2020 09:58:00 -0500 Received: from us-smtp-delivery-1.mimecast.com ([205.139.110.120]:41642 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 1is6aq-0007y4-GZ for qemu-devel@nongnu.org; Thu, 16 Jan 2020 09:57:56 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1579186675; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=i60m9e8tMfEi+qimoetZDnfJap1BHGDS5lxvOYe9I6w=; b=Dfc9ku3xWNuaR66ucEA46EMvORxzkFAqOFXPvUYhi4v850bRgE2tRpvf5i9t90jxW1gVJi fp/wmgoHgWaXxiZnXqI0uMCOl7lHg/MdhzfGahmy7G8es7sWZHZip+0NwqYm9ohih2I+BA CV8h+A9n5cIM5RXgcXk1N82G1WHuMio= 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-18-rHMw8OvnPa2P0QMXZnMKNQ-1; Thu, 16 Jan 2020 09:57:52 -0500 Received: from smtp.corp.redhat.com (int-mx08.intmail.prod.int.phx2.redhat.com [10.5.11.23]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id DBE02113784D; Thu, 16 Jan 2020 14:57:51 +0000 (UTC) Received: from [10.3.117.16] (ovpn-117-16.phx2.redhat.com [10.3.117.16]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 63F2719488; Thu, 16 Jan 2020 14:57:51 +0000 (UTC) Subject: Re: Bug? qemu-img convert to preallocated image makes it sparse To: Max Reitz , "Richard W.M. Jones" , qemu-devel@nongnu.org, qemu-block@nongnu.org, mlevitsk@redhat.com, sgarzare@redhat.com References: <20200116141352.GA32053@redhat.com> <962aa54b-f6e5-bb43-50a0-c4cad59cd22e@redhat.com> From: Eric Blake Organization: Red Hat, Inc. Message-ID: <75daaf8f-0bfe-d3f6-5df4-88c29b2d9b07@redhat.com> Date: Thu, 16 Jan 2020 08:57:50 -0600 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.3.1 MIME-Version: 1.0 In-Reply-To: <962aa54b-f6e5-bb43-50a0-c4cad59cd22e@redhat.com> Content-Language: en-US X-Scanned-By: MIMEDefang 2.84 on 10.5.11.23 X-MC-Unique: rHMw8OvnPa2P0QMXZnMKNQ-1 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: 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: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 1/16/20 8:47 AM, Max Reitz wrote: > So when you convert to the target image, you have to make sure all areas > that are zero in the source are zero in the target, too. The way we do > that is to write zeroes to the target. The problem is that this > operation disregards the previous preallocation and discards the > preallocated space. >=20 > As for fixing the bug... Can we fix it in qemu(-img)? >=20 > We could try to detect whether areas that are zero in the source are > zero in the (preallocated) target image, too. But doing so what require > reading the data from those areas and comparing it to zero. That would > take time and it isn=E2=80=99t trivial. So that=E2=80=99s something I=E2= =80=99d rather avoid. Can't we also use block status queries on the destination, as that is=20 likely to be faster than actual reads and comparison to zero? --=20 Eric Blake, Principal Software Engineer Red Hat, Inc. +1-919-301-3226 Virtualization: qemu.org | libvirt.org