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Thu, 9 Apr 2020 15:01:26 +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> <8b4bc264-7bce-c9c1-1905-a22b4c61cae4@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: <713d39ff-29f6-f9e0-bbbc-c9b26ffd28a0@redhat.com> Date: Thu, 9 Apr 2020 17:01:24 +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: X-Scanned-By: MIMEDefang 2.79 on 10.5.11.11 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="0akCmnqqclApdG8I6NOYgGfo9Q1puGe3W" 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) --0akCmnqqclApdG8I6NOYgGfo9Q1puGe3W Content-Type: multipart/mixed; boundary="QsRABDBkwrxcfcIHMAIvkfD2J0HhuSSQO" --QsRABDBkwrxcfcIHMAIvkfD2J0HhuSSQO Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: quoted-printable On 09.04.20 16:32, Eric Blake wrote: > On 4/9/20 9:10 AM, Max Reitz wrote: >=20 >>> >>> What happens when an operation attempts to unmap things?=C2=A0 Do we re= ject >>> all unmap operations when data-file-raw is set (thus leaving a cluster >>> marked as allocated at all times, if we can first guarantee that >>> preallocation set things up that way)? >> No, unmap operations currently work.=C2=A0 qcow2_free_any_clusters() pas= ses >> them through to the external data file. >> >> The problem is that the unmap also zeroes the L2 entry, so if you then >> write data to the raw file, it won=E2=80=99t be visible from the qcow2 s= ide of >> things.=C2=A0 However, I=E2=80=99m not sure whether we support modificat= ions of a raw >> file when it is already =E2=80=9Cin use=E2=80=9D by a qcow2 image, so ma= ybe that=E2=80=99s fine. >=20 > We don't support concurrent modification. But if the guest is running > and unmaps things, then shuts off, then we edit the raw file offline, > then we restart the guest, the guest should see the results of those > offline edits. Should it? The specification doesn=E2=80=99t say anything about that. In fact, I think we have always said we explicitly discourage that because this might lead to outdated metadata; even though we usually meant =E2=80=9Cdirty bitmaps=E2=80=9D by that. Max --QsRABDBkwrxcfcIHMAIvkfD2J0HhuSSQO-- --0akCmnqqclApdG8I6NOYgGfo9Q1puGe3W Content-Type: application/pgp-signature; name="signature.asc" Content-Description: OpenPGP digital signature Content-Disposition: attachment; filename="signature.asc" -----BEGIN PGP SIGNATURE----- iQEzBAEBCAAdFiEEkb62CjDbPohX0Rgp9AfbAGHVz0AFAl6POMQACgkQ9AfbAGHV z0Atvgf+OqRJixMQXYS0XBJfQ7p+BKDSixMSg4mqTRLIBSuQ65H/MjWaNo2ERg54 TMHCh2Z58r4emVF/puLEax6cYIv78KTUdsUuKsxcz8ewPMaBHyvG4tO0u1St23D0 fsR1K65Htufrwu79e7fytReSe+Zk3rsfxuI+sXXdWmQPU5c/mjQqgdInXTXlDoK2 cdvalj77vXctjtbcBjaVd10ndw2Q3UdOjNePRqB4thucOYiHBQO0Tcq4U4cRX0m9 Ynx+tsEB6fkwmm47BKCRJUrEz864yFISoT2QNNNjBljSV75fpg6VUPk43LRQpvl7 9uuVgDdfrfQbHENoqZwSer6q6puAgQ== =SIWP -----END PGP SIGNATURE----- --0akCmnqqclApdG8I6NOYgGfo9Q1puGe3W--