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Thu, 24 Oct 2019 15:12:10 +0000 (UTC) Received: from dresden.str.redhat.com (ovpn-117-205.ams2.redhat.com [10.36.117.205]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 10E2010A33E2; Thu, 24 Oct 2019 15:12:03 +0000 (UTC) Subject: Re: [PATCH v5 1/4] block: support compressed write at generic layer To: Andrey Shinkevich , Vladimir Sementsov-Ogievskiy , "qemu-devel@nongnu.org" , "qemu-block@nongnu.org" References: <1571603828-185910-1-git-send-email-andrey.shinkevich@virtuozzo.com> <1571603828-185910-2-git-send-email-andrey.shinkevich@virtuozzo.com> <408ef2ab-1f6c-2c9f-ad50-92269c20fb27@redhat.com> <787da788-9aca-2110-a092-b63ef498a9fa@redhat.com> <1088f0ff-d882-083f-705c-95c08bdc486f@redhat.com> <4fd89d2a-94aa-6f2d-ed7d-7633356e5163@redhat.com> <539e7d9e-0cb7-fdb7-f376-1648361ec2f3@virtuozzo.com> <8828d08f-79aa-f3a6-4387-f1423a8b7d70@redhat.com> <26cfb4ed-941a-11b0-c1cf-bf3c7e354487@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: Date: Thu, 24 Oct 2019 17:12:02 +0200 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: <26cfb4ed-941a-11b0-c1cf-bf3c7e354487@virtuozzo.com> X-Scanned-By: MIMEDefang 2.84 on 10.5.11.22 X-Mimecast-Spam-Score: 0 Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="JIWV1lqDSdOyWxr1j8zBjZfd906fqz8zL" 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: "kwolf@redhat.com" , "fam@euphon.net" , Denis Lunev , "armbru@redhat.com" , "stefanha@redhat.com" 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) --JIWV1lqDSdOyWxr1j8zBjZfd906fqz8zL Content-Type: multipart/mixed; boundary="nXwzCthMmdqfmLp2moXhqtdIpJpjKEKXp" --nXwzCthMmdqfmLp2moXhqtdIpJpjKEKXp Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: quoted-printable On 24.10.19 16:07, Andrey Shinkevich wrote: >=20 >=20 > On 24/10/2019 16:48, Max Reitz wrote: >> On 24.10.19 14:56, Andrey Shinkevich wrote: >>> >>> >>> On 24/10/2019 12:34, Max Reitz wrote: >>>> On 22.10.19 15:53, Andrey Shinkevich wrote: >>>> >>>> [...] >>>> >>>>> If the support of COW for compressed writes is found feasible, will i= t >>>>> make a sense to implement? Then this series will follow. >>>> >>>> Hm, what exactly do you mean by support of COW for compressed writes? >>>> >>> >>> I spoke in terms of the commit message with the following ID: >>> >>> b0b6862e5e1a1394e0ab3d5da94ba8b0da8664e2 >>> >>> "qcow2: Fail write_compressed when overwriting data" >>> >>> "...qcow2_write_compressed() doesn't perform COW as it would have to do= ..." >>> >>> So, I suggest that we implement writing compressed data to the allocate= d >>> clusters rather than qcow2_alloc_compressed_cluster_offset() returns th= e >>> error. Particularly, when it comes to NBD server connection failure for >>> writhing a compressed cluster, it may not be rewritten after the >>> connection is restored. >>> Are there any issues with that implementation idea? >> >> Well, the COW in that commit is meant differently, because it refers to >> the COW that=E2=80=99s required when writing to a cluster shared by an i= nternal >> snapshot. >> >> OTOH, you could say that all compressed writes to a cluster that is >> already allocated would need to do COW because we=E2=80=99d always have = to fully >> rewrite that cluster in an RMW cycle. >> >> I don=E2=80=99t see how letting qcow2_alloc_compressed_cluster_offset() = use the >> existing cluster would solve the problem, though. You=E2=80=99d general= ly need >> to allocate a new cluster; or attempt to reuse the existing space in a >> compressed cluster. >> >> Max >> >=20 > Yes, new clusters would be allocated for the compressed RMW and the=20 > existing ones would be reused if possible. It seams to be ineffective=20 > but users are supposed to know what they do. > So, does it worth to check a feasibility of the implementation? I don=E2=80=99t know, Vladimir said that use case wouldn=E2=80=99t be neede= d. I think if the option was there, people would actually use it. But I doubt that anyone misses it sufficiently to warrant the effort. In addition, there=E2=80=99s still VMDK to consider, too. Max --nXwzCthMmdqfmLp2moXhqtdIpJpjKEKXp-- --JIWV1lqDSdOyWxr1j8zBjZfd906fqz8zL Content-Type: application/pgp-signature; name="signature.asc" Content-Description: OpenPGP digital signature Content-Disposition: attachment; filename="signature.asc" -----BEGIN PGP SIGNATURE----- iQEzBAEBCAAdFiEEkb62CjDbPohX0Rgp9AfbAGHVz0AFAl2xv0IACgkQ9AfbAGHV z0DAxwgAxBKm3TKBDPW2H0J4hPoV+A7yDd/Ce7NKEjyNqF9jDphm8YmfnO+y6Il2 fpRzhQUjqUZRfZOaZqAh5zsizspy+6vbezyYlYRBb5ogb4XkCRFJwm8YIFBCShaO nuugg+hk0v60TVEiUAcpw/budtmzurvM7ovZse5yTsTTwVdv4dw/NFN1nZSsNjfY yNY3J/Mh+fCPtjSFOIi7/8KGiLiDn2iIFqAKiWnVejTd3Lwr3plWDz7Zw4J9BnxO Gn5K/PnHIlhpeBYszPiKKP4fuSDXmVTXuW/VVNhZR/z2DRwY0e01NEv01eD1+NPW 5UwlfLyeA7kwHOd2S7pubz4L+k22zg== =KBnj -----END PGP SIGNATURE----- --JIWV1lqDSdOyWxr1j8zBjZfd906fqz8zL--