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Fri, 7 Feb 2020 14:41:53 +0000 (UTC) Subject: Re: [PATCH v3 1/1] qemu-img: Add --target-is-zero to convert To: Vladimir Sementsov-Ogievskiy , Eric Blake , David Edmondson , qemu-devel@nongnu.org References: <20200204095246.1974117-1-david.edmondson@oracle.com> <20200204095246.1974117-2-david.edmondson@oracle.com> <90f3f74b-6154-d7ce-4e0e-ba4422f7da11@redhat.com> <91c3d45b-4c27-d366-6dd9-5c27164cce35@virtuozzo.com> <92ca6082-a3a6-c116-d1cc-e9810280c0c6@redhat.com> <38ac63ec-af49-d9d5-c1d4-e45614b71d4c@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: <570489b5-8d1b-27c4-01d3-0e63130d2c57@redhat.com> Date: Fri, 7 Feb 2020 15:41:51 +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: X-Scanned-By: MIMEDefang 2.79 on 10.5.11.12 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="OoyiFVJMr0NAEOAhwWDux8lAl2YDh7eTb" X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] X-Received-From: 205.139.110.61 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: 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) --OoyiFVJMr0NAEOAhwWDux8lAl2YDh7eTb Content-Type: multipart/mixed; boundary="d2dAZrtJMbSkAl1JONBCYBb5R2jPce645" --d2dAZrtJMbSkAl1JONBCYBb5R2jPce645 Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: quoted-printable On 07.02.20 13:07, Vladimir Sementsov-Ogievskiy wrote: > 07.02.2020 13:33, Max Reitz wrote: >> On 04.02.20 15:23, Eric Blake wrote: >>> On 2/4/20 7:59 AM, Vladimir Sementsov-Ogievskiy wrote: >>> >>>>> I understand that it is safer to have restrictions now and lift them >>>>> later, than to allow use of the option at any time and leave room for >>>>> the user to shoot themselves in the foot with no way to add safety >>>>> later.=C2=A0 The argument against no backing file is somewhat >>>>> understandable (technically, as long as the backing file also reads >>>>> as all zeroes, then the overall image reads as all zeroes - but why >>>>> have a backing file that has no content?); the argument requiring -n >>>>> is a bit weaker (if I'm creating an image, I _know_ it reads as all >>>>> zeroes, so the --target-is-zero argument is redundant, but it >>>>> shouldn't hurt to allow it). >>>> >>>> I know that it reads as all zeroes, only if this format provides zero >>>> initialization.. >>>> >>>>> >>>>>> +++ b/qemu-img.c >>>>> >>>>>> @@ -2247,6 +2256,11 @@ static int img_convert(int argc, char **argv) >>>>>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 warn_re= port("This will become an error in future QEMU >>>>>> versions."); >>>>>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 } >>>>>> +=C2=A0=C2=A0=C2=A0 if (s.has_zero_init && !skip_create) { >>>>>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 error_report("--target-i= s-zero requires use of -n flag"); >>>>>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 goto fail_getopt; >>>>>> +=C2=A0=C2=A0=C2=A0 } >>>>> >>>>> So I think we could drop this hunk with no change in behavior. >>>> >>>> I think, no we can't. If we allow target-is-zero, with -n, we'd better >>>> to check that what we are creating is zero-initialized (format has >>>> zero-init), and if not we should report error. >>>> >>> >>> Good call.=C2=A0 Yes, if we allow --target-is-zero without -n, we MUST = insist >>> that bdrv_has_zero_init() returns 1 (or, after my followup series, >>> bdrv_known_zeroes() includes BDRV_ZERO_CREATE). >> >> Why? >> >> I could imagine a user creating a qcow2 image on some block device with >> preallocation where we cannot verify that the result will be zero.=C2=A0= But >> they want qemu not to zero the device, so they would specify >> --target-is-zero. >=20 > If user create image, setting --target-is-zero is always valid. But if > we in > same operation create the image automatically, having --target-is-zero, > when > we know that what we are creating is not zero is misleading and should > fail.. bdrv_has_zero_init() doesn=E2=80=99t return false only for images that we k= now are not zero. It returns true for images where we know they are. But if we don=E2=80=99t know, then it returns false also. > If we want to add a behavior to skip zeros unconditionally, we should > call new > option --skip-zeroes, to clearly specify what we want. It was my impression that this was exactly what --target-is-zero means and implies. 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