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Thu, 2 Jul 2020 14:50:06 +0000 (UTC) Subject: Re: [PATCH v9 30/34] qcow2: Add prealloc field to QCowL2Meta To: Alberto Garcia , qemu-devel@nongnu.org References: <0dd88b3b4d3e90b28267267d7686cf5350d9dea0.1593342067.git.berto@igalia.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 16:50:04 +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: <0dd88b3b4d3e90b28267267d7686cf5350d9dea0.1593342067.git.berto@igalia.com> 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="6uOnktM6KUYLY1arRk1FT3lSuIZyhRJbY" 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) --6uOnktM6KUYLY1arRk1FT3lSuIZyhRJbY Content-Type: multipart/mixed; boundary="kbpCIoZiCAQT0FL08PU9abhIcxGj3sy9n" --kbpCIoZiCAQT0FL08PU9abhIcxGj3sy9n Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: quoted-printable 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. >=20 > 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. >=20 > This will allow preallocating images that have a backing file. >=20 > 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). >=20 > In this case the subclusters cannot be left as unallocated so the L2 > bitmap must be updated. >=20 > Signed-off-by: Alberto Garcia > Reviewed-by: Eric Blake > --- > block/qcow2.h | 8 ++++++++ > block/qcow2-cluster.c | 2 +- > block/qcow2.c | 6 ++++++ > 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. That=E2=80=99s not ideal, and avoiding those COWs may be a reason to do preallocation in the first place. Now, with backing files, it=E2=80=99s entirely correct. 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. Of course, if we did that, you couldn=E2=80=99t create the overlay separate= ly from the backing file, preallocate it, and only then attach the backing file to the overlay. But is that a problem? (Or are there other problems to consider?) Max --kbpCIoZiCAQT0FL08PU9abhIcxGj3sy9n-- --6uOnktM6KUYLY1arRk1FT3lSuIZyhRJbY Content-Type: application/pgp-signature; name="signature.asc" Content-Description: OpenPGP digital signature Content-Disposition: attachment; filename="signature.asc" -----BEGIN PGP SIGNATURE----- iQEzBAEBCAAdFiEEkb62CjDbPohX0Rgp9AfbAGHVz0AFAl799BwACgkQ9AfbAGHV z0C1NggAiLgSQ9iLHc5kLGt9k7kaW4PWqdwjZBRbmfQHQ5z1WBZUVD5gFzCdbixa i/DTKumqSYiulvVFe1oyvwNnmTZDyFIW0Zu6c7t1ynp7ZZRB0Cuj3TmR7XtMiGqJ 1D5kO7c2kVC9Hqsgot1vyMqQfoVhUKHxq26IpWYTuPGhHPyt1kaznmqA8AIxcSDD ziuccGvy3kCDexaUObWNiSg2x/kvhIysoSdMR8ftO/vAXGk5aMm5Nacfkb4lK+rM yYzWwvoFo4AJipFpeDL81QvcC37kTau36vi0WojUpcbfK+iEzJ5NYbowAymVwvOV cFgZigHOEmggsatO1zNN9HqCzJ/tgg== =FYdA -----END PGP SIGNATURE----- --6uOnktM6KUYLY1arRk1FT3lSuIZyhRJbY--