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Thu, 21 Nov 2019 08:51:53 +0000 (UTC) Subject: Re: qcow2 preallocation and backing files To: Vladimir Sementsov-Ogievskiy , Alberto Garcia , "qemu-devel@nongnu.org" References: <20191120120625.GA25497@igalia.com> <5b57fb5b-4480-2b39-9c60-bbd63f13f1cb@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: <78235737-2182-f126-af84-151b1e01b803@redhat.com> Date: Thu, 21 Nov 2019 09:51:50 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.1.1 MIME-Version: 1.0 In-Reply-To: <5b57fb5b-4480-2b39-9c60-bbd63f13f1cb@virtuozzo.com> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.16 X-Mimecast-Spam-Score: 0 Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="g510DDqpNQ65IvlolNyvakwKV1pQbRzON" X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] [fuzzy] 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: Kevin Wolf , "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) --g510DDqpNQ65IvlolNyvakwKV1pQbRzON Content-Type: multipart/mixed; boundary="upATMp6S9iD6mPkgRfGK514WqNu8GAgmb" --upATMp6S9iD6mPkgRfGK514WqNu8GAgmb Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: quoted-printable On 20.11.19 16:58, Vladimir Sementsov-Ogievskiy wrote: > 20.11.2019 18:18, Alberto Garcia wrote: >> On Wed 20 Nov 2019 01:27:53 PM CET, Vladimir Semeeausntsov-Ogievskiy wro= te: >> >>> 3. Also, the latter way is inconsistent with discard. Discarded >>> regions returns zeroes, not clusters from backing. I think discard and >>> truncate should behave in the same safe zero way. >> >> But then PREALLOC_MODE_OFF implies that the L2 metadata should be >> preallocated (all clusters should be QCOW2_CLUSTER_ZERO_PLAIN), at least >> when there is a backing file. >> >> Or maybe we just forbid PREALLOC_MODE_OFF during resize if there is a >> backing file ? >> >=20 > Kevin proposed a fix that alters PREALLOC_MODE_OFF behavior if there is > a backing file, to allocate L2 metadata with ZERO clusters.. >=20 > I don't think that it's the best thing to do, but it's already done, it w= orks > and seems appropriate for rc3.. >=20 > I see now, that change PREALLOC_MODE_OFF behavior may break things, first= of > all qemu-img create, which creating UNALLOCATED qcow2 by default for year= s. >=20 > Still, I think that it would be safer to always ZERO expanded part of qco= w2, > regardless of backing file.. >=20 > We may add PREALLOC_MODE_ZERO, and use it in mirror, commit, and some oth= er calls > to bdrv_truncate, except for qcow2 image creation of course. Well, the good news is that block_resize currently has no such parameter and thus we could make a non-OFF value the default. The bad news is that the reason it has no such parameter is that it would need to be a job in order to support any form of preallocation. So actually I don=E2=80=99t think we can make block_resize write zeroes to = the new region by default, because then it needs to be a job, and it just isn=E2=80=99t. (Of course, we can get to it through a deprecation cycle, b= ut we can=E2=80=99t =E2=80=9Cfix=E2=80=9D block_resize directly.) Max --upATMp6S9iD6mPkgRfGK514WqNu8GAgmb-- --g510DDqpNQ65IvlolNyvakwKV1pQbRzON Content-Type: application/pgp-signature; name="signature.asc" Content-Description: OpenPGP digital signature Content-Disposition: attachment; filename="signature.asc" -----BEGIN PGP SIGNATURE----- iQEzBAEBCAAdFiEEkb62CjDbPohX0Rgp9AfbAGHVz0AFAl3WUCcACgkQ9AfbAGHV z0CFgwf8Ci1dsKKJbL5LxaQca14LmK4vqBgGf40RDFCv++0OuK4H2RpplIGxi3uN LI/bdtaGIqohQ/zwNMEviskeITcoArw9TKYhoxla/pyemz84XS4+KC1jVxK29aQM yqoDTcb3qoGmen9lFifcPOORSvIXFjqu8jfeD1KHA0YG0XWSI3HzSY49CHcbFWKd 5EO1hPu08nssfar1B0dRRmBrWwhiZb37budfZDJlqOVmhGY0eHvJ8hzJ43yXzHJb fLiw5s1+PVPI+fEgIUyUewdzn8npn7hu+oPHxvKwnrhCVtWoEKwo7XJtGmZoijlI AKF4wxavJFlNtr7VvWU6MWvitX9h0A== =W34d -----END PGP SIGNATURE----- --g510DDqpNQ65IvlolNyvakwKV1pQbRzON--