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,INCLUDES_PATCH,MAILING_LIST_MULTI,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 D5AA6C35247 for ; Wed, 5 Feb 2020 17:06:56 +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 8B3C821741 for ; Wed, 5 Feb 2020 17:06:56 +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="GWy0f6bZ" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 8B3C821741 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]:53456 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1izO8d-0003Nm-Q8 for qemu-devel@archiver.kernel.org; Wed, 05 Feb 2020 12:06:55 -0500 Received: from eggs.gnu.org ([2001:470:142:3::10]:59980) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1izO6p-0001Tr-FN for qemu-devel@nongnu.org; Wed, 05 Feb 2020 12:05:05 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1izO6n-0003PO-GT for qemu-devel@nongnu.org; Wed, 05 Feb 2020 12:05:03 -0500 Received: from us-smtp-delivery-1.mimecast.com ([205.139.110.120]:48366 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 1izO6n-0003Lb-9v for qemu-devel@nongnu.org; Wed, 05 Feb 2020 12:05:01 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1580922300; 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: in-reply-to:in-reply-to:references:references:autocrypt:autocrypt; bh=yd/jhEUd1j9qzAH4ivPNsvj2GD9Ypk8jOVY4BN+/cuU=; b=GWy0f6bZPNg6ilttr28JSlBMc2uoi9ST+AoC9dboZC5IaEckuYfraWZnW0G3CaQBKb03+f ReQ3jg0emhiH+emzUCqMekZRsecr/g3IY8KPeINT6cf+JmMSWpaToxX1gW1/GrDYE9ThBb Oc/tg62ZrZFRHvBNZOaVu9anWI7UiaE= 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-66-NTja_AliNdCUbfJNpw_0Fw-1; Wed, 05 Feb 2020 12:04:19 -0500 X-MC-Unique: NTja_AliNdCUbfJNpw_0Fw-1 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 5C9F7801A0B; Wed, 5 Feb 2020 17:04:18 +0000 (UTC) Received: from dresden.str.redhat.com (ovpn-117-184.ams2.redhat.com [10.36.117.184]) by smtp.corp.redhat.com (Postfix) with ESMTPS id F3B9D19C7F; Wed, 5 Feb 2020 17:04:16 +0000 (UTC) Subject: Re: [PATCH 00/17] Improve qcow2 all-zero detection To: Eric Blake , qemu-devel@nongnu.org References: <20200131174436.2961874-1-eblake@redhat.com> <8574b42d-479e-ef72-ecab-4546b364adb6@redhat.com> From: Max Reitz Autocrypt: addr=mreitz@redhat.com; prefer-encrypt=mutual; keydata= mQENBFXOJlcBCADEyyhOTsoa/2ujoTRAJj4MKA21dkxxELVj3cuILpLTmtachWj7QW+TVG8U /PsMCFbpwsQR7oEy8eHHZwuGQsNpEtNC2G/L8Yka0BIBzv7dEgrPzIu+W3anZXQW4702+uES U29G8TP/NGfXRRHGlbBIH9KNUnOSUD2vRtpOLXkWsV5CN6vQFYgQfFvmp5ZpPeUe6xNplu8V mcTw8OSEDW/ZnxJc8TekCKZSpdzYoxfzjm7xGmZqB18VFwgJZlIibt1HE0EB4w5GsD7x5ekh awIe3RwoZgZDLQMdOitJ1tUc8aqaxvgA4tz6J6st8D8pS//m1gAoYJWGwwIVj1DjTYLtABEB AAG0HU1heCBSZWl0eiA8bXJlaXR6QHJlZGhhdC5jb20+iQFTBBMBCAA9AhsDBQkSzAMABQsJ CAcCBhUICQoLAgQWAgMBAh4BAheABQJVzie5FRhoa3A6Ly9rZXlzLmdudXBnLm5ldAAKCRD0 B9sAYdXPQDcIB/9uNkbYEex1rHKz3mr12uxYMwLOOFY9fstP5aoVJQ1nWQVB6m2cfKGdcRe1 2/nFaHSNAzT0NnKz2MjhZVmcrpyd2Gp2QyISCfb1FbT82GMtXFj1wiHmPb3CixYmWGQUUh+I AvUqsevLA+WihgBUyaJq/vuDVM1/K9Un+w+Tz5vpeMidlIsTYhcsMhn0L9wlCjoucljvbDy/ 8C9L2DUdgi3XTa0ORKeflUhdL4gucWoAMrKX2nmPjBMKLgU7WLBc8AtV+84b9OWFML6NEyo4 4cP7cM/07VlJK53pqNg5cHtnWwjHcbpGkQvx6RUx6F1My3y52vM24rNUA3+ligVEgPYBuQEN BFXOJlcBCADAmcVUNTWT6yLWQHvxZ0o47KCP8OcLqD+67T0RCe6d0LP8GsWtrJdeDIQk+T+F xO7DolQPS6iQ6Ak2/lJaPX8L0BkEAiMuLCKFU6Bn3lFOkrQeKp3u05wCSV1iKnhg0UPji9V2 W5eNfy8F4ZQHpeGUGy+liGXlxqkeRVhLyevUqfU0WgNqAJpfhHSGpBgihUupmyUg7lfUPeRM DzAN1pIqoFuxnN+BRHdAecpsLcbR8sQddXmDg9BpSKozO/JyBmaS1RlquI8HERQoe6EynJhd 64aICHDfj61rp+/0jTIcevxIIAzW70IadoS/y3DVIkuhncgDBvGbF3aBtjrJVP+5ABEBAAGJ ASUEGAEIAA8FAlXOJlcCGwwFCRLMAwAACgkQ9AfbAGHVz0CbFwf9F/PXxQR9i4N0iipISYjU sxVdjJOM2TMut+ZZcQ6NSMvhZ0ogQxJ+iEQ5OjnIputKvPVd5U7WRh+4lF1lB/NQGrGZQ1ic alkj6ocscQyFwfib+xIe9w8TG1CVGkII7+TbS5pXHRxZH1niaRpoi/hYtgzkuOPp35jJyqT/ /ELbqQTDAWcqtJhzxKLE/ugcOMK520dJDeb6x2xVES+S5LXby0D4juZlvUj+1fwZu+7Io5+B bkhSVPb/QdOVTpnz7zWNyNw+OONo1aBUKkhq2UIByYXgORPFnbfMY7QWHcjpBVw9MgC4tGeF R4bv+1nAMMxKmb5VvQCExr0eFhJUAHAhVg== Message-ID: <5141ea4b-a7c2-e9a3-045e-91dc088785c7@redhat.com> Date: Wed, 5 Feb 2020 18:04:14 +0100 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: <8574b42d-479e-ef72-ecab-4546b364adb6@redhat.com> X-Scanned-By: MIMEDefang 2.84 on 10.5.11.23 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="bbrfyNICn9TlPH6BTZk6DQFFOVCPfjFjH" X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] 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: , Cc: david.edmondson@oracle.com, qemu-block@nongnu.org Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" This is an OpenPGP/MIME signed message (RFC 4880 and 3156) --bbrfyNICn9TlPH6BTZk6DQFFOVCPfjFjH Content-Type: multipart/mixed; boundary="T5hFsJSifO0Diy6vsKmN6X5a11m4DYhkp" --T5hFsJSifO0Diy6vsKmN6X5a11m4DYhkp Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: quoted-printable On 04.02.20 19:53, Eric Blake wrote: > 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.=C2=A0 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.=C2=A0 The NBD patch will come later (and cross-poste= d to >>> the NBD protocol, libnbd, nbdkit, and qemu, as it will affect all four >>> repositories). >> >> 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.=C2=A0 So, again, I wonder why t= his is needed. >> >> 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=9C= We don=E2=80=99t >> know=E2=80=9D as an answer.=C2=A0 So I=E2=80=99m looking for use cases, = but this cover letter >> doesn=E2=80=99t mention any.=C2=A0 (And from a quick glance I don=E2=80= =99t see this series >> using the flag, actually.) >=20 > Patch 10/17 has: >=20 > 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) > =C2=A0=C2=A0=C2=A0=C2=A0 int64_t sector_num =3D 0; >=20 > =C2=A0=C2=A0=C2=A0=C2=A0 /* Check whether we have zero initialisation or = can get it > efficiently */ > -=C2=A0=C2=A0=C2=A0 if (!s->has_zero_init && s->target_is_new && s->min_s= parse && > -=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 !s->target_has_backing) { > -=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 s->has_zero_init =3D !!(bdrv_= known_zeroes(blk_bs(s->target)) & > -=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0= =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2= =A0=C2=A0=C2=A0=C2=A0=C2=A0 BDRV_ZERO_CREATE); > +=C2=A0=C2=A0=C2=A0 if (!s->has_zero_init && s->min_sparse && !s->target_= has_backing) { > +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 ret =3D bdrv_known_zeroes(blk= _bs(s->target)); > +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 if (ret & BDRV_ZERO_OPEN || > +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 (s->t= arget_is_new && ret & BDRV_ZERO_CREATE)) { > +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 s->ha= s_zero_init =3D true; > +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 } > =C2=A0=C2=A0=C2=A0=C2=A0 } OK, I expected users to come in a separate patch. > That's the use case: when copying into a destination file, it's useful > to know if the destination already reads as all zeroes, before > attempting a fallback to bdrv_make_zero(BDRV_REQ_NO_FALLBACK) or calls > to block status checking for holes. But that was my point on IRC. Is it really more useful if bdrv_make_zero() is just as quick? (And the fact that NBD doesn=E2=80=99t = have an implementation looks more like a problem with NBD to me.) (Considering that at least the code we discussed on IRC didn=E2=80=99t work= for preallocated images, which was the one point where we actually have a problem in practice.) >> (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.=C2=A0 Well, management >> layers, because I don=E2=80=99t think users would use convert -n anyway. >> >> 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.) >=20 > Yes, there will still be cases where you have to use --target-is-zero > because the image itself couldn't report that it already reads as > zeroes, but there are also enough cases where the destination is already > known to read zeroes and it's a shame to tell the user that 'you have to > add --target-is-zero to get faster copying even though we could have > inferred it on your behalf'. How is it a shame? I think only management tools would use convert -n. Management tools want reliable behavior. If you want reliable behavior, you have to use --target-is-zero anyway. So I don=E2=80=99t see = the actual benefit for qemu-img convert. >> I suppose there is the point of blockdev-create + blockdev-mirror: This >> has exactly the same problem as convert -n.=C2=A0 But again, if you real= ly >> 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). >> >> (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.) >> >> But maybe I=E2=80=99m just an idiot and there is no reason not to take t= his >> series and make blockdev-create + blockdev-mirror do the sensible thing >> by default in most cases. *shrug* >=20 > My argument for taking the series _is_ that the common case can be made > more efficient without user effort. The thing is, I don=E2=80=99t see the user effort. I don=E2=80=99t think u= sers use convert -n or backup manually. And for management tools, it isn=E2=80=99t really effort to add another switch. > Yes, we still need the knob for > when the common case isn't already smart enough, But the user can=E2=80=99t know when qemu isn=E2=80=99t smart enough. So u= sers who care have to always give the flag. > but the difference in > avoiding a pre-zeroing pass is noticeable when copying images around I=E2=80=99m sure it is, but the question I ask is whether in practice we wouldn=E2=80=99t get --target-is-zero in all of these cases anyway. So I=E2=80=99m not sold on =E2=80=9Cit works most of the time=E2=80=9D, bec= ause if it=E2=80=99s just most of the time, then we=E2=80=99ll likely see --target-is-zero all of the= time. OTOH, I suppose that with the new qcow2 extension, it would always work for the following case: (1) Create a qcow2 file, (2) Immediately (with the next qemu-img/QMP invocation) use it as a target of convert -n or mirror or anything similar. If so, that means it works reliably all of the time for a common case. I guess that=E2=80=99d be enough for me. Max > (and more than just for qcow2 - my followup series to improve NBD is > similarly useful given how much work has already been invested in > mapping NBD into storage access over https in the upper layers like ovirt= ). >=20 --T5hFsJSifO0Diy6vsKmN6X5a11m4DYhkp-- --bbrfyNICn9TlPH6BTZk6DQFFOVCPfjFjH Content-Type: application/pgp-signature; name="signature.asc" Content-Description: OpenPGP digital signature Content-Disposition: attachment; filename="signature.asc" -----BEGIN PGP SIGNATURE----- iQEzBAEBCAAdFiEEkb62CjDbPohX0Rgp9AfbAGHVz0AFAl469Y4ACgkQ9AfbAGHV z0CQZgf/ZCOUt8PZcxpSuSjY58jEkTGMoFGb9eURBf3Q1cNrO/ZiAlx9ASjkgaBq ENJzqNpH4M9f0yu0iFYfbTEFVcq4+pSdWzS6kGYDgbe86/4hmaSC5SvSPu7C0M77 kk79PFqK9BeuKg5gI5JxxvSlnUZZP4RMvPfXAGRz1kU59WBVj4BguvBsM9NUysbl SdBuzbmvsZj3+y0ctpjfxdNhDAkaxkpdBcj7MFoBKBL6yeWlXqq/8x4zgwGRkISH +rlHXaz2+HjrOuLtqeMzdVZ6D5tK9fcrCtXPxHefhnshjE3a23DM9e0UT2DiiScU e06OkU4org/zfexUyUtEhN3UIM5R3A== =CY8O -----END PGP SIGNATURE----- --bbrfyNICn9TlPH6BTZk6DQFFOVCPfjFjH--