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=-5.1 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, 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 209DCC33CB1 for ; Thu, 16 Jan 2020 14:55:11 +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 E44CB207E0 for ; Thu, 16 Jan 2020 14:55:10 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="QLlFohpe" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org E44CB207E0 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]:43430 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1is6YA-00040F-0a for qemu-devel@archiver.kernel.org; Thu, 16 Jan 2020 09:55:10 -0500 Received: from eggs.gnu.org ([2001:470:142:3::10]:56605) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1is6XF-0002k2-20 for qemu-devel@nongnu.org; Thu, 16 Jan 2020 09:54:14 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1is6XA-0004ou-Uf for qemu-devel@nongnu.org; Thu, 16 Jan 2020 09:54:12 -0500 Received: from us-smtp-1.mimecast.com ([207.211.31.81]:47209 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 1is6XA-0004oG-Qf for qemu-devel@nongnu.org; Thu, 16 Jan 2020 09:54:08 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1579186447; 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=LkTpGjSngT+63eT11k7EYoND9omNu8vZmjzQZ3GmGOc=; b=QLlFohpet6tCUYgpr9Ds1gyS5AgK/DBNNav7h2QxwpFACV7Wan1cupeO28BIvFSpuNHnju YBKHgDHTngW+sEj81iGHArVAcQAMAQDkyVAXloXdCso1YUFBGRawKWc8wj68tpSNmBBAMm AAmUfTYmvOjk/6W3cSPfHX9ZldgKmYE= 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-115-qFrH3xoRNmagJAQaMFYshw-1; Thu, 16 Jan 2020 09:54:01 -0500 Received: from smtp.corp.redhat.com (int-mx02.intmail.prod.int.phx2.redhat.com [10.5.11.12]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id A7BA0107ACC9; Thu, 16 Jan 2020 14:54:00 +0000 (UTC) Received: from localhost (ovpn-117-237.ams2.redhat.com [10.36.117.237]) by smtp.corp.redhat.com (Postfix) with ESMTP id 4B44160C85; Thu, 16 Jan 2020 14:54:00 +0000 (UTC) Date: Thu, 16 Jan 2020 14:53:59 +0000 From: "Richard W.M. Jones" To: Max Reitz Subject: Re: Bug? qemu-img convert to preallocated image makes it sparse Message-ID: <20200116145359.GR3888@redhat.com> References: <20200116141352.GA32053@redhat.com> <962aa54b-f6e5-bb43-50a0-c4cad59cd22e@redhat.com> MIME-Version: 1.0 In-Reply-To: <962aa54b-f6e5-bb43-50a0-c4cad59cd22e@redhat.com> User-Agent: Mutt/1.5.21 (2010-09-15) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.12 X-MC-Unique: qFrH3xoRNmagJAQaMFYshw-1 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable Content-Disposition: inline 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: ptoscano@redhat.com, sgarzare@redhat.com, qemu-devel@nongnu.org, qemu-block@nongnu.org, mlevitsk@redhat.com Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Thu, Jan 16, 2020 at 03:47:30PM +0100, Max Reitz wrote: > On 16.01.20 15:13, Richard W.M. Jones wrote: > > I'm not necessarily saying this is a bug, but a change in behaviour in > > qemu has caused virt-v2v to fail. The reproducer is quite simple. > >=20 > > Create sparse and preallocated qcow2 files of the same size: > >=20 > > $ qemu-img create -f qcow2 sparse.qcow2 50M > > Formatting 'sparse.qcow2', fmt=3Dqcow2 size=3D52428800 cluster_size= =3D65536 lazy_refcounts=3Doff refcount_bits=3D16 > >=20 > > $ qemu-img create -f qcow2 prealloc.qcow2 50M -o preallocation=3Dfall= oc,compat=3D1.1 > > Formatting 'prealloc.qcow2', fmt=3Dqcow2 size=3D52428800 compat=3D1.1= cluster_size=3D65536 preallocation=3Dfalloc lazy_refcounts=3Doff refcount_= bits=3D16 > >=20 > > $ du -m sparse.qcow2 prealloc.qcow2=20 > > 1 sparse.qcow2 > > 51=09prealloc.qcow2 > >=20 > > Now copy the sparse file into the preallocated file using the -n > > option so qemu-img doesn't create the target: > >=20 > > $ qemu-img convert -p -n -f qcow2 -O qcow2 sparse.qcow2 prealloc.qcow= 2 > > (100.00/100%) > >=20 > > In new qemu that makes the target file sparse: > >=20 > > $ du -m sparse.qcow2 prealloc.qcow2=20 > > 1 sparse.qcow2 > > 1 prealloc.qcow2 <-- should still be 51 > >=20 > > In old qemu the target file remained preallocated, which is what > > I and virt-v2v are expecting. > >=20 > > I bisected this to the following commit: > >=20 > > 4d7c487eac1652dfe4498fe84f32900ad461d61b is the first bad commit > > commit 4d7c487eac1652dfe4498fe84f32900ad461d61b > > Author: Max Reitz > > Date: Wed Jul 24 19:12:29 2019 +0200 > >=20 > > qemu-img: Fix bdrv_has_zero_init() use in convert > > =20 > > bdrv_has_zero_init() only has meaning for newly created images or i= mage > > areas. If qemu-img convert did not create the image itself, it can= not > > rely on bdrv_has_zero_init()'s result to carry any meaning. > > =20 > > Signed-off-by: Max Reitz > > Message-id: 20190724171239.8764-2-mreitz@redhat.com > > Reviewed-by: Maxim Levitsky > > Reviewed-by: Stefano Garzarella > > Signed-off-by: Max Reitz > >=20 > > qemu-img.c | 11 ++++++++--- > > 1 file changed, 8 insertions(+), 3 deletions(-) > >=20 > > Reverting this commit on the current master branch restores the > > expected behaviour. >=20 > So what this commit changed was that when you take an existing image as > the destination, you can=E2=80=99t assume anything about its contents. B= efore > this commit, we assumed it=E2=80=99s zero. That=E2=80=99s clearly wrong,= because it can > be anything. >=20 > 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. >=20 > Off the top of my head, the only thing that comes to my mind would be to > add a flag to qemu-img convert with which you can let it know that you > guarantee the target image is zero. I suppose we could document it also > to imply that given this flag, areas that are zero in the source will > then not be changed in the target image; i.e. that preallocation stays > intact in those areas. >=20 >=20 > OTOH, can it be fixed in virt-v2v? Is there already a safe way to call > qemu-img convert -n and keeping the target=E2=80=99s preallocation intact= ? > Unfortunately, I don=E2=80=99t think so. I don=E2=80=99t think we ever g= uaranteed it > would, and well, now it broke. >From the fixing virt-v2v point of view, it's a bit tricky since the code has to deal with all kinds of output targets. (For example we sometimes qemu-img convert into an NBD target.) However we do know when the target contains zeroes - in fact it always contains zeroes, so: > So would you be OK with a --target-is-zero flag? (I think we could let > this flag guarantee that your use case works, so it should be future-safe= .) this one should work. Rich. --=20 Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjon= es Read my programming and virtualization blog: http://rwmj.wordpress.com Fedora Windows cross-compiler. Compile Windows programs, test, and build Windows installers. Over 100 libraries supported. http://fedoraproject.org/wiki/MinGW