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Thu, 2 Jul 2020 15:09:48 +0000 (UTC) Subject: Re: [PATCH v9 30/34] qcow2: Add prealloc field to QCowL2Meta To: Eric Blake , Alberto Garcia , qemu-devel@nongnu.org References: <0dd88b3b4d3e90b28267267d7686cf5350d9dea0.1593342067.git.berto@igalia.com> <687f5402-2316-e06a-2fd1-37375a4fa119@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: Date: Thu, 2 Jul 2020 17:09:47 +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: <687f5402-2316-e06a-2fd1-37375a4fa119@redhat.com> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.16 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="HdFlgJKSlTkC4UQbumK6M1IlEeZ4vPMSu" Received-SPF: pass client-ip=207.211.31.81; envelope-from=mreitz@redhat.com; helo=us-smtp-delivery-1.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/07/02 04:18:28 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 , Derek Su , Vladimir Sementsov-Ogievskiy , 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) --HdFlgJKSlTkC4UQbumK6M1IlEeZ4vPMSu Content-Type: multipart/mixed; boundary="gBbKa85tsaw1OY0EIl3UmxdPH4BuWiV5g" --gBbKa85tsaw1OY0EIl3UmxdPH4BuWiV5g Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: quoted-printable On 02.07.20 16:58, Eric Blake wrote: > On 7/2/20 9:50 AM, Max Reitz wrote: >> On 28.06.20 13:02, Alberto Garcia wrote: >>> This field allows us to indicate that the L2 metadata update does not >>> come from a write request with actual data but from a preallocation >>> request. >>> >>> For traditional images this does not make any difference, but for >>> images with extended L2 entries this means that the clusters are >>> allocated normally in the L2 table but individual subclusters are >>> marked as unallocated. >>> >>> This will allow preallocating images that have a backing file. >>> >>> There is one special case: when we resize an existing image we can >>> also request that the new clusters are preallocated. If the image >>> already had a backing file then we have to hide any possible stale >>> data and zero out the new clusters (see commit 955c7d6687 for more >>> details). >>> >>> In this case the subclusters cannot be left as unallocated so the L2 >>> bitmap must be updated. >>> >>> Signed-off-by: Alberto Garcia >>> Reviewed-by: Eric Blake >>> --- >>> =C2=A0 block/qcow2.h=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 | = 8 ++++++++ >>> =C2=A0 block/qcow2-cluster.c | 2 +- >>> =C2=A0 block/qcow2.c=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 | = 6 ++++++ >>> =C2=A0 3 files changed, 15 insertions(+), 1 deletion(-) >> >> Sounds good, but I=E2=80=99m just not quite sure about the details on >> falloc/full allocation: With .prealloc =3D true, writing to the >> preallocated subclusters will require a COW operation.=C2=A0 That=E2=80= =99s not >> ideal, and avoiding those COWs may be a reason to do preallocation in >> the first place. >=20 > I'm not sure I follow the complaint.=C2=A0 If a cluster is preallocated b= ut > the subcluster is marked unallocated, then doing a partial write to that > subcluster must provide the correct contents for the rest of the > subcluster (either filling with zero, or reading from a backing file) - > but this COW can be limited to just the portion of the subcluster, and > is no different than the COW you have to perform without subclusters > when doing a write to a preallocated cluster in general. It was my impression that falloc/full preallocation would create normal data clusters, not zero clusters, so no COW was necessary when writing to them. >> Now, with backing files, it=E2=80=99s entirely correct.=C2=A0 You need a= COW >> operation, because that=E2=80=99s the point of having a backing file. >> >> But without a backing file I wonder if it wouldn=E2=80=99t be better to = set >> .prealloc =3D false to avoid that COW. >=20 > Without a backing file, there is no read required - writing to an > unallocated subcluster within a preallocated cluster merely has to > provide zeros to the rest of the write.=C2=A0 And depending on whether we= can > intelligently guarantee that the underlying protocol already reads as > zeroes when preallocated, we even have an optimization where even that > is not necessary.=C2=A0 We can still lump it in the "COW" terminology, in > that our write is more complex than merely writing in place, but it > isn't a true copy-on-write operation as there is nothing to be copied. The term =E2=80=9CCOW=E2=80=9D specifically in the qcow2 driver also refers= to having to write zeroes to an area that isn=E2=80=99t written to by the guest as part = of the process of having to allocate a (sub)cluster. (Of course there is no COW from a backing file if there is no backing file.= ) Max --gBbKa85tsaw1OY0EIl3UmxdPH4BuWiV5g-- --HdFlgJKSlTkC4UQbumK6M1IlEeZ4vPMSu Content-Type: application/pgp-signature; name="signature.asc" Content-Description: OpenPGP digital signature Content-Disposition: attachment; filename="signature.asc" -----BEGIN PGP SIGNATURE----- iQEzBAEBCAAdFiEEkb62CjDbPohX0Rgp9AfbAGHVz0AFAl79+LsACgkQ9AfbAGHV z0A+zgf+PY5V3idD3Ln0vlcdyUiEarWY7/+B5ee9dHpMxUSW7z7wDJtzS9x5hhHH 036IDxFxtMO617c/wwuZyfVD/gSKSfay+z3IsMRcCNnO1VK4XdJ9XS9yx5/awEw6 asRndmLSMB2WMVBrQxA3Fbdyrpz9Lataz51QADAfz63xOYZ2R/YhHu8f2hsSXKxO lt+hcgq4RR8r0FQWAhtWuj2dHm2BJw4QVrJbpH27H4alN3BCiAmW5GW1PjO7dihv DE/gu656Yjd3rKsn0q0/iLndCI+VJHIDB9cWnKE5fwW7fydCiNY4+xmzduARh4b6 RnVbXl95XShKAlVtC6KbuJ24OIzBaQ== =ATnD -----END PGP SIGNATURE----- --HdFlgJKSlTkC4UQbumK6M1IlEeZ4vPMSu--