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Mon, 22 Jun 2020 15:06:55 +0000 (UTC) Subject: Re: [PATCH 1/2] qcow2: Force preallocation with data-file-raw To: Alberto Garcia , qemu-block@nongnu.org References: <20200619104012.235977-1-mreitz@redhat.com> <20200619104012.235977-2-mreitz@redhat.com> <9c14c622-eb30-4619-d33a-b59395a397be@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: <2d35fdff-6230-18b9-cf99-ca72be53267f@redhat.com> Date: Mon, 22 Jun 2020 17:06:53 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.9.0 MIME-Version: 1.0 In-Reply-To: X-Scanned-By: MIMEDefang 2.79 on 10.5.11.12 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=mreitz@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="1heR9friC98AwAjyajeb2SItLunR7Av4e" Received-SPF: pass client-ip=205.139.110.120; envelope-from=mreitz@redhat.com; helo=us-smtp-1.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/06/22 01:27:42 X-ACL-Warn: Detected OS = Linux 2.2.x-3.x [generic] [fuzzy] X-Spam_score_int: -30 X-Spam_score: -3.1 X-Spam_bar: --- X-Spam_report: (-3.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-1, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H3=-0.01, RCVD_IN_MSPIKE_WL=-0.01, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=_AUTOLEARN X-Spam_action: no action 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: Kevin Wolf , 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) --1heR9friC98AwAjyajeb2SItLunR7Av4e Content-Type: multipart/mixed; boundary="jWsr8VPYzW27NwfZPf3d2so2N6kr2yqQU" --jWsr8VPYzW27NwfZPf3d2so2N6kr2yqQU Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: quoted-printable On 22.06.20 16:46, Alberto Garcia wrote: > On Mon 22 Jun 2020 11:35:59 AM CEST, Max Reitz wrote: >>>> + if (qcow2_opts->data_file_raw && >>>> + qcow2_opts->preallocation =3D=3D PREALLOC_MODE_OFF) >>>> + { >>>> + /* >>>> + * data-file-raw means that "the external data file can be >>>> + * read as a consistent standalone raw image without looking >>>> + * at the qcow2 metadata." It does not say that the metadata >>>> + * must be ignored, though (and the qcow2 driver in fact does >>>> + * not ignore it), so the L1/L2 tables must be present and >>>> + * give a 1:1 mapping, so you get the same result regardless >>>> + * of whether you look at the metadata or whether you ignore >>>> + * it. >>>> + */ >>>> + qcow2_opts->preallocation =3D PREALLOC_MODE_METADATA; >>> >>> I'm not convinced by this, >> >> Why not? >> >> This is how I read the spec. Furthermore, I see two problems that we >> have right now that are fixed by this patch (namely (1) using a device >> file as the external data file, which may have non-zero data at >> creation; and (2) assigning a backing file at runtime must not show >> the data). >=20 > What happens if you first create the image (which would be preallocated > with this patch), then you resize it and finally you assign the backing > file? Would the resized part be preallocated? Good point, when resizing an image with data-file-raw we also need to preallocate the L2 tables. >>> but your comment made me think of another possible alternative: in >>> qcow2_get_cluster_offset(), if the cluster is unallocated and we are >>> using a raw data file then we return _ZERO_PLAIN: >>> >>> --- a/block/qcow2-cluster.c >>> +++ b/block/qcow2-cluster.c >>> @@ -654,6 +654,10 @@ out: >>> assert(bytes_available - offset_in_cluster <=3D UINT_MAX); >>> *bytes =3D bytes_available - offset_in_cluster; >>> =20 >>> + if (type =3D=3D QCOW2_CLUSTER_UNALLOCATED && data_file_is_raw(bs))= { >>> + type =3D QCOW2_CLUSTER_ZERO_PLAIN; >>> + } >>> + >>> return type; >>> >>> You could even add a '&& bs->backing' to the condition and emit a >>> warning to make it more explicit. >> >> No, this is wrong. This still wouldn=E2=80=99t fix the problem of havin= g a >> device file as the external data file, when it already has non-zero >> data during creation. (Reading the qcow2 file would return zeroes, >> but reading the device would not.) >=20 > But you wouldn't fix that preallocating the metadata either, you would > need to fill the device with zeroes. What it fixes is that reading the qcow2 image and the raw device returns the same data. Initially, I also thought that we should initialize raw data files to be zero during creation, but Eric changed my mind: https://lists.nongnu.org/archive/html/qemu-block/2020-04/msg00223.html >> I interpret the spec in that the metadata can be ignored, but it does >> not need to be ignored. So the L1/L2 tables must be 1:1 mapping of >> QCOW2_CLUSTER_NORMAL entries. >> >> We could also choose to interpret it as =E2=80=9CWith data-file-raw, the= L1/L2 >> tables must be ignored=E2=80=9D. In that case, our qcow2 driver would n= eed to >> be modified to indeed fully ignore the L1/L2 tables with >> data-file-raw. (I certainly don=E2=80=99t interpret the spec this way, = but I >> suppose we could call it a bug fix and amend it.) >=20 > The way I interpret it is that regardless of whether you read the data > through the qcow2 file or directly from the data file you should get the > same results, but how that should be reflected in the L1/L2 metadata is > not specified. That=E2=80=99s an absolute given, but the question is what does =E2=80=9Cre= ading through the qcow2 file=E2=80=9D mean. Respecting the metadata? Ignoring it? Some= thing in between? As I noted in my reply to myself, data-file-raw is an autoclear flag. That means, an old version of qemu that doesn=E2=80=99t recognize the flag = must read the same data as a new version. It follows that the the L2 tables must be a 1:1 mapping. (Or the flag can=E2=80=99t be an autoclear flag.) 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