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Tue, 4 Feb 2020 17:32:46 +0000 (UTC) Received: from dresden.str.redhat.com (ovpn-117-98.ams2.redhat.com [10.36.117.98]) by smtp.corp.redhat.com (Postfix) with ESMTPS id AAAC45D9C5; Tue, 4 Feb 2020 17:32:44 +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> 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: Date: Tue, 4 Feb 2020 18:32:42 +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: <20200131174436.2961874-1-eblake@redhat.com> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.14 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="oLwNroRkPMMIbwqmQMyjWISP5QBa41am6" 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" This is an OpenPGP/MIME signed message (RFC 4880 and 3156) --oLwNroRkPMMIbwqmQMyjWISP5QBa41am6 Content-Type: multipart/mixed; boundary="bIKhK8YLNHAK6rSaWbMR3oiJme3gri01J" --bIKhK8YLNHAK6rSaWbMR3oiJme3gri01J Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: quoted-printable 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) >=20 > 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). 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 n= eeded. 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 ca= se where we would need to figure it out and where we could accept =E2=80=9CWe = don=E2=80=99t know=E2=80=9D as an answer. So I=E2=80=99m looking for use cases, but this= cover letter doesn=E2=80=99t mention any. (And from a quick glance I don=E2=80=99t see = this series using the flag, actually.) (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. 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.) 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). (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 this series and make blockdev-create + blockdev-mirror do the sensible thing by default in most cases. *shrug* Max --bIKhK8YLNHAK6rSaWbMR3oiJme3gri01J-- --oLwNroRkPMMIbwqmQMyjWISP5QBa41am6 Content-Type: application/pgp-signature; name="signature.asc" Content-Description: OpenPGP digital signature Content-Disposition: attachment; filename="signature.asc" -----BEGIN PGP SIGNATURE----- iQEzBAEBCAAdFiEEkb62CjDbPohX0Rgp9AfbAGHVz0AFAl45qroACgkQ9AfbAGHV z0BrsQf/c09Rqmm+qoVxM3crwz3Lrt04bIqU4636RaZmN7tX5gquAAvJ0m6V4J18 94xwEbYWVEvVCbc3ffVbIB7r/KZ0pzEhTlBGY95U5yKyOhEAaJmftqtVOqzSy9wd k+B1EHVFmxadJMSJNStddFxx8Ze1IDeq1JUFVZYdezf4jfb7MGe1VYpPQoFwT4a0 zK7lB+6h+9h43h/jaUxy6Z2lkbt63NWHHKRClhrfZrrw0e93j8SJnaLV9LE+z99w 1yauq9zTzEC+bVzUXQxXGZ0ayh/nXF3lIVrkCAy54gdR6AxBu7PgbwwOwx4YugQH 4qOUHxszbik+37GgK08Yqm5nBXJN2g== =jL/p -----END PGP SIGNATURE----- --oLwNroRkPMMIbwqmQMyjWISP5QBa41am6--