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Fri, 7 Feb 2020 15:28:28 +0000 (UTC) Received: from dresden.str.redhat.com (ovpn-117-14.ams2.redhat.com [10.36.117.14]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 25FF460BF7; Fri, 7 Feb 2020 15:28:26 +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> <570489b5-8d1b-27c4-01d3-0e63130d2c57@redhat.com> <99dc03a7-f93a-bdee-f38d-ac5aae8a3ca9@virtuozzo.com> <33849f7f-1ae9-126c-28b8-f9a5602aa2d0@redhat.com> <96aab671-23db-1917-983e-182312a41220@virtuozzo.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: Fri, 7 Feb 2020 16:28:25 +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: <96aab671-23db-1917-983e-182312a41220@virtuozzo.com> 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="M8Z60bh6sZYGAIN3yuXxVk0qW7R07TxSz" 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) --M8Z60bh6sZYGAIN3yuXxVk0qW7R07TxSz Content-Type: multipart/mixed; boundary="FO6nnQaZcymSDgyN6j4c84w5UtqnFIwYr" --FO6nnQaZcymSDgyN6j4c84w5UtqnFIwYr Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: quoted-printable On 07.02.20 16:16, Vladimir Sementsov-Ogievskiy wrote: > 07.02.2020 18:03, Max Reitz wrote: >> On 07.02.20 15:53, Vladimir Sementsov-Ogievskiy wrote: >>> 07.02.2020 17:41, Max Reitz wrote: >>>> 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 safet= y >>>>>>>>> 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=C2= =A0=C2=A0 warn_report("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=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("--targ= et-is-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 M= UST >>>>>>> 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. >>>>> >>>>> If user create image, setting --target-is-zero is always valid. But i= f >>>>> 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 shoul= d >>>>> fail.. >>>> >>>> bdrv_has_zero_init() doesn=E2=80=99t return false only for images that= we know >>>> are not zero.=C2=A0 It returns true for images where we know they are.= =C2=A0 But >>>> if we don=E2=80=99t know, then it returns false also. >>> >>> yes, but we don't have better check. >> >> Correct, but maybe the user knows more, hence why it may make sense for >> them to provide us with some information we don=E2=80=99t have. >> >>>>> 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. >>>> >>> >>> For me it sounds strange that user has better knowledge about what Qemu >>> creates than Qemu itself. And if it so - it should be fixed in Qemu, >>> rather than creating user interface to hint Qemu what it does. >> >> I brought an example where qemu cannot know whether the image is zero >> (preallocation on a block device), but the user / management layer might >> know. >> >=20 > It sounds unsafe for me. User can't know how exactly Qemu do preallocatio= n, > which syscalls it calls, etc. How can user be sure, that Qemu produces > all-zero image, if even Qemu doesn't sure in it? For example, qcow2 images are always zero if the underlying storage is zero= . Then again, it isn=E2=80=99t like we need --target-is-zero without -n anywa= y. If users want that, they can always use qemu-img create + qemu-img convert -n --target-is-zero. So seeing that you=E2=80=99re uncomfortable with the idea of --target-is-zero with -n, I can=E2=80=99t sa= y there is any actual reason why we=E2=80=99d have to allow it. Max > Otherwise, we should document, how exactly (up to syscalls, their > parameters, flags, the whole logic and algorithm) preallocation is done, > so user can analyze it and be sure that produced image would be all-zero > (when Qemu can't determine it because some specific block device, for whi= ch > Qemu doesn't know that its preallocation algorithm produces all-zero, but > user is sure in it).. --FO6nnQaZcymSDgyN6j4c84w5UtqnFIwYr-- --M8Z60bh6sZYGAIN3yuXxVk0qW7R07TxSz Content-Type: application/pgp-signature; name="signature.asc" Content-Description: OpenPGP digital signature Content-Disposition: attachment; filename="signature.asc" -----BEGIN PGP SIGNATURE----- iQEzBAEBCAAdFiEEkb62CjDbPohX0Rgp9AfbAGHVz0AFAl49ghkACgkQ9AfbAGHV z0B1ywf/cBMI/ZRf6f3KLANuv1q5wjVqpeoNlYoAhBVeZ9cQy3ODNsS2HLRbHawY uL7PAtBeUrp+fLLE7UbpEC15heuL+DshgPxtI+3jaZ1W6eI29VbRr5KCkBHiaSG6 o7QmX6ovzQH+yP6nU8YLqMWEEoA1a/LMUJFcd3mN7/Z9rwOP/MCbGFyZhV5mtxHo grgz90cD1vIwRrejGa/N1xn8sRUyryzYp5uTEqLz5BHlb7au38YI4uTxb1GDkfOo sqId2/HRogOTGhECnkHl8Rdw8cOxIVwBKf+xCTG17dWE4oQ96VWUa12nU/YY4spB dhA4JgmPdZOwUSCqJf6NNnumqowdEg== =5xrY -----END PGP SIGNATURE----- --M8Z60bh6sZYGAIN3yuXxVk0qW7R07TxSz--