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Thu, 9 Apr 2020 13:05:13 +0000 (UTC) Received: from dresden.str.redhat.com (ovpn-114-98.ams2.redhat.com [10.36.114.98]) by smtp.corp.redhat.com (Postfix) with ESMTPS id DE6FB5D9E5; Thu, 9 Apr 2020 13:05:11 +0000 (UTC) Subject: Re: qcow2: Zero-initialization of external data files To: Eric Blake , Qemu-block , Kevin Wolf References: <50080252-ff22-78ed-0002-1742c694471b@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: <8b4bc264-7bce-c9c1-1905-a22b4c61cae4@redhat.com> Date: Thu, 9 Apr 2020 15:05:09 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.6.0 MIME-Version: 1.0 In-Reply-To: <50080252-ff22-78ed-0002-1742c694471b@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="MLzLY8gT8UKDqPDDIOm10T7K1kAm2Oiai" 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: "qemu-devel@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) --MLzLY8gT8UKDqPDDIOm10T7K1kAm2Oiai Content-Type: multipart/mixed; boundary="QpP9iWVKcHdctget9Hu5tSGfQnYxNEBGn" --QpP9iWVKcHdctget9Hu5tSGfQnYxNEBGn Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: quoted-printable On 07.04.20 00:22, Eric Blake wrote: > On 2/17/20 10:56 AM, Max Reitz wrote: >> Hi, >> >> AFAIU, external data files with data_file_raw=3Don are supposed to retur= n >> the same data as the qcow2 file when read.=C2=A0 But we still use the qc= ow2 >> metadata structures (which are by default initialized to =E2=80=9Ceveryt= hing >> unallocated=E2=80=9D), even though we never ensure that the external dat= a file >> is zero, too, so this can happen: >> >> $ dd if=3D/dev/urandom of=3Dfoo.raw 64M >> [...] >> >> $ sudo losetup -f --show foo.raw >> /dev/loop0 >> >> $ sudo ./qemu-img create -f qcow2 -o \ >> =C2=A0=C2=A0=C2=A0=C2=A0 data_file=3D/dev/loop0,data_file_raw=3Don foo.q= cow2 64M >> [...] >> >> $ sudo ./qemu-io -c 'read -P 0 0 64M' foo.qcow2 >> read 67108864/67108864 bytes at offset 0 >> 64 MiB, 1 ops; 00.00 sec (25.036 GiB/sec and 400.5751 ops/sec) >=20 > This looks like a bug (and we should fix it for 5.0 if possible) It seems a bit difficult for 5.0 now. (But I don=E2=80=99t think it=E2=80= =99d be a regression, so that shouldn=E2=80=99t be too bad.) > read > of a data_file_raw=3Don should not treat unallocated clusters as reading > 0, but rather as reading whatever the raw data contains. >=20 >> >> $ sudo ./qemu-io -c 'read -P 0 0 64M' -f raw foo.raw >> Pattern verification failed at offset 0, 67108864 bytes >> read 67108864/67108864 bytes at offset 0 >> 64 MiB, 1 ops; 00.01 sec (5.547 GiB/sec and 88.7484 ops/sec) >> >> I suppose this behavior is fine for blockdev-create because I guess it= =E2=80=99s >> the user=E2=80=99s responsibility to ensure that the external data file = is zero. >> =C2=A0 But maybe it isn=E2=80=99t, so that=E2=80=99s my first question: = Is it really the >> user=E2=80=99s responsibility or should we always ensure it=E2=80=99s ze= ro? >=20 > I'd argue that requiring the user to pre-zero the raw data file is > undesirable; and that we should instead fix our code to not report the > image as reading all zeroes when creating with data_file_raw=3Don. OK. I think that could be achieved by just enforcing @preallocation to be at least =E2=80=9Cmetadata=E2=80=9D whenever @data-file-raw is set. Wou= ld that make sense? Max >> My second question is: If we decide that this is fine for >> blockdev-create, should at least qcow2_co_create_opts() ensure the data >> file it just created is zero? >=20 > Having an option to make qemu force-zero the raw image during > qcow2_co_create_opts seems reasonable, but for performance reasons, I > don't think the flag should be on by default. >=20 --QpP9iWVKcHdctget9Hu5tSGfQnYxNEBGn-- --MLzLY8gT8UKDqPDDIOm10T7K1kAm2Oiai Content-Type: application/pgp-signature; name="signature.asc" Content-Description: OpenPGP digital signature Content-Disposition: attachment; filename="signature.asc" -----BEGIN PGP SIGNATURE----- iQEzBAEBCAAdFiEEkb62CjDbPohX0Rgp9AfbAGHVz0AFAl6PHYUACgkQ9AfbAGHV z0AyiQgAsXAmSHQpPfJME9djs9vhL7HfvsqPuzxru+D5E2A0TCOkRuAS+HXHweuI Izv8AxyM5g8y6tW0+yGwkyEi0HaICuKPYbSkwq9+zBo9bcL+evPEx495FxMER+yt dA8sUiIXpM/X+Y4HuBT17056uSY/D/WCgoz0zFuxhUCnW04TbCdX+I2xXSkaTshO 01c1WaZ/PMlCwTAx79iDn70EScqeiNhqWBlCIzvCOTg7RBc17930BxPzL0KEnKvt ZVn5ILfPw5qNgi/ikRkdCeAfysqGUeeAHylcZOkU3xP5Zv4Fq3eE2XHgSqQBVcmH cIWp+vmPa8qeykONEsQcahUnWG17xQ== =wWe1 -----END PGP SIGNATURE----- --MLzLY8gT8UKDqPDDIOm10T7K1kAm2Oiai--