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Tue, 23 Jun 2020 07:28:08 +0000 (UTC) Subject: Re: [PATCH 0/2] qcow2: Force preallocation with data-file-raw To: Alberto Garcia , Nir Soffer References: <20200619104012.235977-1-mreitz@redhat.com> <72eb5700-faf6-22ee-69be-1e7653cf06c6@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: <66581529-dc44-5fd0-54a3-1117b073077a@redhat.com> Date: Tue, 23 Jun 2020 09:28:07 +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 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="seCU8vtQ3X1sUkaVbz7EQs3Fr8YeQaCna" 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 = 2020/06/23 01:53:54 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 Developers , qemu-block 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) --seCU8vtQ3X1sUkaVbz7EQs3Fr8YeQaCna Content-Type: multipart/mixed; boundary="iIECAPImeEA1VTCPjlTKLtSXHceUQc3DE" --iIECAPImeEA1VTCPjlTKLtSXHceUQc3DE Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: quoted-printable On 22.06.20 19:44, Alberto Garcia wrote: > On Mon 22 Jun 2020 11:47:32 AM CEST, Max Reitz wrote: >>> I don't know the internals of qcow2 data_file, but are we really using >>> qcow2 metadata when accessing the data file? >> >> Yes. >> >>> This may have unwanted performance consequences. >> >> I don=E2=80=99t think so, because in practice normal lookups of L1/L2 ma= ppings >> generally don=E2=80=99t cost that much performance. >=20 > ...if the L2 cache size is large enough. Otherwise you need one extra > read operation to retrieve the L2 metadata. >=20 > Possible performance problems when you have preallocation: >=20 > - If a block hasn't been written yet (it's all zeroes) then you still > need to read the L2 entry and read the data block. If there is not > L2 table then you can simply return zeroes without going to disk at > all. This of course assumes that the contents of the unwritten data > block are zeroes. >=20 > - QEMU still needs to read from disk (and cache in memory) the L2 > metadata, when it already knows in advance the contents of the L2 > entry (guest_offset =3D=3D host_offset). We could well optimize this regardless of preallocation. With data-file-raw, qemu doesn=E2=80=99t have to look at the L2 metadata at all. So the problem isn=E2=80=99t preallocation at all, it=E2=80=99s the fact th= at we don=E2=80=99t have such an optimization. But note that to implement such an optimization, we really do need preallocation: Because it would mean that we wouldn=E2=80=99t touch the L1/L2 tables for data-file-raw images du= ring runtime, which would effectively make those images empty to today=E2=80=99s= qemu versions. (OTOH, preallocation would then be pretty much superfluous for all newer versions of qemu. To address that, we could then add an incompatible version of data-file-raw. But I think we should only think about that once we get to that point.) You make a good point that data-file-raw was introduced alongside data-file. But, well. I personally can=E2=80=99t get myself to treating a= n autoclear flag like an incompatible one... Max --iIECAPImeEA1VTCPjlTKLtSXHceUQc3DE-- --seCU8vtQ3X1sUkaVbz7EQs3Fr8YeQaCna Content-Type: application/pgp-signature; name="signature.asc" Content-Description: OpenPGP digital signature Content-Disposition: attachment; filename="signature.asc" -----BEGIN PGP SIGNATURE----- iQEzBAEBCAAdFiEEkb62CjDbPohX0Rgp9AfbAGHVz0AFAl7xrwcACgkQ9AfbAGHV z0Aokwf/Wya+AwABVfK+TbjFXpvXBW0pN44MZoq99nDHRxX8QRB90XCEzd77+2LZ HB2TFan9WwtSuJ+3lHy0SbKUVn6sJrTC5wGREHdOCElMsFMVHCZOQ9canLhrv+PK sLNhB+mICZSFhVzaLZ4nwgkm6qpdXNAfgC0kZjtf/lkjArxdXvlr6T17jBlrFZEH ULCyHjd1F7fYKQOK+Qs8XxDxzM9BZclwGNQEmPSv42VMaXWTfOw8ZQFooD53hxgn NRvt85d/YBQSS+GY3Ug9oY93e1me+LI5LdP4jTIt+rQSP+qAQdsgTc9Xye/x2Jyh PGeXu30hNnY3woG6zhfcTESQrF5qvw== =Y2E7 -----END PGP SIGNATURE----- --seCU8vtQ3X1sUkaVbz7EQs3Fr8YeQaCna--