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=-5.0 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS,URIBL_BLOCKED,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 A54BFC35247 for ; Tue, 4 Feb 2020 18:54:03 +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 615CB2082E for ; Tue, 4 Feb 2020 18:54:03 +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="Qq6cJZgC" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 615CB2082E 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]:36412 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1iz3Kk-0006YA-H6 for qemu-devel@archiver.kernel.org; Tue, 04 Feb 2020 13:54:02 -0500 Received: from eggs.gnu.org ([2001:470:142:3::10]:58786) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1iz3K1-0005w7-1t for qemu-devel@nongnu.org; Tue, 04 Feb 2020 13:53:18 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1iz3Jy-0000Ul-NT for qemu-devel@nongnu.org; Tue, 04 Feb 2020 13:53:16 -0500 Received: from us-smtp-delivery-1.mimecast.com ([207.211.31.120]:26488 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 1iz3Jy-0000Rx-J7 for qemu-devel@nongnu.org; Tue, 04 Feb 2020 13:53:14 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1580842393; 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=PvQUVGEpWZtnw5ZVuOyuLfl4ut8rV3zCC5kFU1P8938=; b=Qq6cJZgC9hCDauorol9WEIY25U4m6495bU/Xrckr2Wfeg9xYZ0hfukptww8axpYg4810iB yRoyrkN0O7YyEt7duw4WrvVnhy/7gkY2QnH8PTgFlQrPK0mS264Q4qO9IZ/RRRiHCEB6bX Dy5QZ5g4FyJyXc7vkVIjdMe8BcdqJu0= 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-343-g1rKJgBMN8-V0gcfGtQbjQ-1; Tue, 04 Feb 2020 13:53:09 -0500 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 78C871085925; Tue, 4 Feb 2020 18:53:08 +0000 (UTC) Received: from [10.3.116.181] (ovpn-116-181.phx2.redhat.com [10.3.116.181]) by smtp.corp.redhat.com (Postfix) with ESMTPS id D6C725D9E2; Tue, 4 Feb 2020 18:53:07 +0000 (UTC) Subject: Re: [PATCH 00/17] Improve qcow2 all-zero detection To: Max Reitz , qemu-devel@nongnu.org References: <20200131174436.2961874-1-eblake@redhat.com> From: Eric Blake Organization: Red Hat, Inc. Message-ID: <8574b42d-479e-ef72-ecab-4546b364adb6@redhat.com> Date: Tue, 4 Feb 2020 12:53:07 -0600 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.4.1 MIME-Version: 1.0 In-Reply-To: Content-Language: en-US X-Scanned-By: MIMEDefang 2.79 on 10.5.11.14 X-MC-Unique: g1rKJgBMN8-V0gcfGtQbjQ-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: 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: david.edmondson@oracle.com, qemu-block@nongnu.org Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 2/4/20 11:32 AM, Max Reitz wrote: > On 31.01.20 18:44, Eric Blake wrote: >> Based-on: <20200124103458.1525982-2-david.edmondson@oracle.com> >> ([PATCH v2 1/2] qemu-img: Add --target-is-zero to convert) >> >> I'm working on adding an NBD extension that reports whether an image >> is already all zero when the client first connects. I initially >> thought I could write the NBD code to just call bdrv_has_zero_init(), >> but that turned out to be a bad assumption that instead resulted in >> this patch series. The NBD patch will come later (and cross-posted to >> the NBD protocol, libnbd, nbdkit, and qemu, as it will affect all four >> repositories). >=20 > We had a discussion about this on IRC, and as far as I remember I wasn=E2= =80=99t > quite sold on the =E2=80=9Cwhy=E2=80=9D. So, again, I wonder why this is= needed. >=20 > I mean, it does make intuitive sense to want to know whether an image is > fully zero, but if I continue thinking about it I don=E2=80=99t know any = case > where we would need to figure it out and where we could accept =E2=80=9CW= e don=E2=80=99t > know=E2=80=9D as an answer. So I=E2=80=99m looking for use cases, but th= is cover letter > doesn=E2=80=99t mention any. (And from a quick glance I don=E2=80=99t se= e this series > using the flag, actually.) Patch 10/17 has: diff --git a/qemu-img.c b/qemu-img.c index e60217e6c382..c8519a74f738 100644 --- a/qemu-img.c +++ b/qemu-img.c @@ -1985,10 +1985,12 @@ static int convert_do_copy(ImgConvertState *s) int64_t sector_num =3D 0; /* Check whether we have zero initialisation or can get it=20 efficiently */ - if (!s->has_zero_init && s->target_is_new && s->min_sparse && - !s->target_has_backing) { - s->has_zero_init =3D !!(bdrv_known_zeroes(blk_bs(s->target)) & - BDRV_ZERO_CREATE); + if (!s->has_zero_init && s->min_sparse && !s->target_has_backing) { + ret =3D bdrv_known_zeroes(blk_bs(s->target)); + if (ret & BDRV_ZERO_OPEN || + (s->target_is_new && ret & BDRV_ZERO_CREATE)) { + s->has_zero_init =3D true; + } } That's the use case: when copying into a destination file, it's useful=20 to know if the destination already reads as all zeroes, before=20 attempting a fallback to bdrv_make_zero(BDRV_REQ_NO_FALLBACK) or calls=20 to block status checking for holes. >=20 > (We have a use case with convert -n to freshly created image files, but > my position on this on IRC was that we want the --target-is-zero flag > for that anyway: Auto-detection may always break, our preferred default > behavior may always change, so if you want convert -n not to touch the > target image except to write non-zero data from the source, we need a > --target-is-zero flag and users need to use it. Well, management > layers, because I don=E2=80=99t think users would use convert -n anyway. >=20 > And with --target-is-zero and users effectively having to use it, I > don=E2=80=99t think that=E2=80=99s a good example of a use case.) Yes, there will still be cases where you have to use --target-is-zero=20 because the image itself couldn't report that it already reads as=20 zeroes, but there are also enough cases where the destination is already=20 known to read zeroes and it's a shame to tell the user that 'you have to=20 add --target-is-zero to get faster copying even though we could have=20 inferred it on your behalf'. >=20 > I suppose there is the point of blockdev-create + blockdev-mirror: This > has exactly the same problem as convert -n. But again, if you really > want blockdev-mirror not just to force-zero the image, you probably need > to tell it so explicitly (i.e., with a --target-is-zero flag for > blockdev-mirror). >=20 > (Well, I suppose we could save us a target-is-zero for mirror if we took > this series and had a filter driver that force-reports BDRV_ZERO_OPEN. > But, well, please no.) >=20 > But maybe I=E2=80=99m just an idiot and there is no reason not to take th= is > series and make blockdev-create + blockdev-mirror do the sensible thing > by default in most cases. *shrug* My argument for taking the series _is_ that the common case can be made=20 more efficient without user effort. Yes, we still need the knob for=20 when the common case isn't already smart enough, but the difference in=20 avoiding a pre-zeroing pass is noticeable when copying images around=20 (and more than just for qcow2 - my followup series to improve NBD is=20 similarly useful given how much work has already been invested in=20 mapping NBD into storage access over https in the upper layers like ovirt). --=20 Eric Blake, Principal Software Engineer Red Hat, Inc. +1-919-301-3226 Virtualization: qemu.org | libvirt.org