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Mon, 4 Nov 2019 14:21:45 +0000 (UTC) Received: from dresden.str.redhat.com (unknown [10.36.118.44]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 264DC60863; Mon, 4 Nov 2019 14:21:42 +0000 (UTC) Subject: Re: [RFC PATCH v2 13/26] qcow2: Add subcluster support to calculate_l2_meta() To: Alberto Garcia , qemu-devel@nongnu.org References: <4cf84e3126dc87486b19514c211d46941cdff243.1572125022.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: <5a45821e-564f-957e-5121-e11db318d214@redhat.com> Date: Mon, 4 Nov 2019 15:21:41 +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: <4cf84e3126dc87486b19514c211d46941cdff243.1572125022.git.berto@igalia.com> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.13 X-Mimecast-Spam-Score: 0 Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="YcrAFJneS2UuVjpzwUysxeEAUIo7DkHIu" X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] [fuzzy] X-Received-From: 207.211.31.120 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 , Anton Nefedov , qemu-block@nongnu.org, Vladimir Sementsov-Ogievskiy , "Denis V . Lunev" 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) --YcrAFJneS2UuVjpzwUysxeEAUIo7DkHIu Content-Type: multipart/mixed; boundary="aZOAs8RQsidui1E6NLoOF1iS5aIbibXQm" --aZOAs8RQsidui1E6NLoOF1iS5aIbibXQm Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: quoted-printable On 26.10.19 23:25, Alberto Garcia wrote: > If an image has subclusters then there are more copy-on-write > scenarios that we need to consider. Let's say we have a write request > from the middle of subcluster #3 until the end of the cluster: >=20 > - If the cluster is new, then subclusters #0 to #3 from the old > cluster must be copied into the new one. You mean for snapshots? (That isn=E2=80=99t quite clear, and I only guess this based on the next bu= llet point which differentiates based on =E2=80=9Cthe old cluster was unallocate= d=E2=80=9D. That=E2=80=99s weird, too, because what does that mean, old cluster and new cluster? I suppose it=E2=80=99s abstract and it just means =E2=80=9CThere = was no old cluster and now we=E2=80=99ve allocated something=E2=80=9D. I can only und= erstand the concept of old and new clusters for COW inside of an image, i.e. for snapshots and compressed clusters (theoretically).) > - If the cluster is new but the old cluster was unallocated, then > only subcluster #3 needs copy-on-write. #0 to #2 are marked as > unallocated in the bitmap of the new L2 entry. >=20 > - If we are overwriting an old cluster and subcluster #3 is > unallocated or has the all-zeroes bit set then we need > copy-on-write on subcluster #3. >=20 > - If we are overwriting an old cluster and subcluster #3 was > allocated then there is no need to copy-on-write. >=20 > Signed-off-by: Alberto Garcia > --- > block/qcow2-cluster.c | 136 +++++++++++++++++++++++++++++++++--------- > 1 file changed, 108 insertions(+), 28 deletions(-) >=20 > diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c > index 1f509bda15..990bc070af 100644 > --- a/block/qcow2-cluster.c > +++ b/block/qcow2-cluster.c > @@ -1034,14 +1034,16 @@ void qcow2_alloc_cluster_abort(BlockDriverState *= bs, QCowL2Meta *m) > * If @keep_old is true it means that the clusters were already > * allocated and will be overwritten. If false then the clusters are > * new and we have to decrease the reference count of the old ones. > + * > + * Returns 1 on success, -errno on failure. I think there should be a note here on why this doesn=E2=80=99t follow the general 0/-errno schema (i.e., =E2=80=9C, because that is what callers gene= rally expect=E2=80=9D). > */ [...] > + if (!keep_old) { > + switch (type) { > + case QCOW2_CLUSTER_NORMAL: > + case QCOW2_CLUSTER_COMPRESSED: > + case QCOW2_CLUSTER_ZERO_ALLOC: > + case QCOW2_CLUSTER_UNALLOCATED_SUBCLUSTER: > + cow_start_from =3D 0; Somehow (I don=E2=80=99t know why) I find this a bit tough to understand. Wouldn=E2=80=99t it work to let cow_start start from the first subcluster f= or ZERO_ALLOC and UNALLOCATED_SUBCLUSTER? We don=E2=80=99t need to COW those,= it should be sufficient to just make the subclusters before that zero or unallocated, respectively. (Same for cow_end) Max > + break; > + case QCOW2_CLUSTER_ZERO_PLAIN: > + case QCOW2_CLUSTER_UNALLOCATED: > + cow_start_from =3D sc_index << s->subcluster_bits; > + break; > + default: > + g_assert_not_reached(); > + } --aZOAs8RQsidui1E6NLoOF1iS5aIbibXQm-- --YcrAFJneS2UuVjpzwUysxeEAUIo7DkHIu Content-Type: application/pgp-signature; name="signature.asc" Content-Description: OpenPGP digital signature Content-Disposition: attachment; filename="signature.asc" -----BEGIN PGP SIGNATURE----- iQEzBAEBCAAdFiEEkb62CjDbPohX0Rgp9AfbAGHVz0AFAl3AM/UACgkQ9AfbAGHV z0A2mgf/UNWaqppOa8kSeyxmsgsK5PJbMLiKgeEszhHDoFNMSFYZN8N7BYmfdcoJ 164uZkPBd0ESxZfl/QFbfFL+DJOD7d8G+Qm+mLQhNMaQnedTrnHXzRs8+iS/p2oj DNpC3gyV9fdiOVGV24RP2KAwx9HMxtg4wcVfZ1q0IeGeONO0PBqRJbYDsZCMjigZ gDy+pSIndAFgDlSRABjAQPZMKD/6MccLgPuj9GMVzyH9hxAb3YNW2NwrGFaJT3qn TGj23oz1VDJ7KkvasgIBiTf8kXxsx9STToqZ3Zqy65gcHzYXoA4IsiOcefIvCBbm LCFjH3LFGDZ0+xzEB8g3htKhXGiPmQ== =c6bP -----END PGP SIGNATURE----- --YcrAFJneS2UuVjpzwUysxeEAUIo7DkHIu--