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Wed, 6 Nov 2019 13:52:07 +0000 (UTC) Received: from dresden.str.redhat.com (ovpn-117-212.ams2.redhat.com [10.36.117.212]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 60C4D60F8A; Wed, 6 Nov 2019 13:52:05 +0000 (UTC) Subject: Re: backup_calculate_cluster_size does not consider source To: Dietmar Maurer , Wolfgang Bumiller References: <1767781109.66.1572948164492@webmail.proxmox.com> <20191106083222.GA189998@stefanha-x1.localdomain> <20191106103450.cafwk7m5xd5eulxo@olga.proxmox.com> <37f72cb7-7085-3c40-7728-e41d59137b3b@redhat.com> <1868807950.27.1573039080274@webmail.proxmox.com> <6684852e-da7d-13b2-f226-1c0074e4ab3b@redhat.com> <396057714.35.1573045777293@webmail.proxmox.com> <2bd155fe-af04-05f2-0bd4-28e844564fc4@redhat.com> <1098165569.40.1573047245058@webmail.proxmox.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: <62bb451e-6b8a-d842-e07c-9f78a6971450@redhat.com> Date: Wed, 6 Nov 2019 14:52:03 +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: <1098165569.40.1573047245058@webmail.proxmox.com> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.12 X-Mimecast-Spam-Score: 0 Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="Lpsgh7rRSgpOeBjYODGB10gn03uyUqGuW" X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] [fuzzy] X-Received-From: 205.139.110.61 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 , Stefan Hajnoczi , "qemu-devel@nongnu.org" , 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) --Lpsgh7rRSgpOeBjYODGB10gn03uyUqGuW Content-Type: multipart/mixed; boundary="vHu5uIYB1yeVaGFkYDv8m47LIHQwfsTjW" --vHu5uIYB1yeVaGFkYDv8m47LIHQwfsTjW Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: quoted-printable On 06.11.19 14:34, Dietmar Maurer wrote: >=20 >> On 6 November 2019 14:17 Max Reitz wrote: >> >> =20 >> On 06.11.19 14:09, Dietmar Maurer wrote: >>>> Let me elaborate: Yes, a cluster size generally means that it is most >>>> =E2=80=9Cefficient=E2=80=9D to access the storage at that size. But t= here=E2=80=99s a tradeoff. >>>> At some point, reading the data takes sufficiently long that reading = a >>>> bit of metadata doesn=E2=80=99t matter anymore (usually, that is). >>> >>> Any network storage suffers from long network latencies, so it always >>> matters if you do more IOs than necessary. >> >> Yes, exactly, that=E2=80=99s why I=E2=80=99m saying it makes sense to me= to increase the >> buffer size from the measly 64 kB that we currently have. I just don=E2= =80=99t >> see the point of increasing it exactly to the source cluster size. >> >>>> There is a bit of a problem with making the backup copy size rather >>>> large, and that is the fact that backup=E2=80=99s copy-before-write ca= uses guest >>>> writes to stall. So if the guest just writes a bit of data, a 4 MB >>>> buffer size may mean that in the background it will have to wait for 4 >>>> MB of data to be copied.[1] >>> >>> We use this for several years now in production, and it is not a proble= m. >>> (Ceph storage is mostly on 10G (or faster) network equipment). >> >> So you mean for cases where backup already chooses a 4 MB buffer size >> because the target has that cluster size? >=20 > To make it clear. Backups from Ceph as source are slow. Yep, but if the target would be another ceph instance, the backup buffer size would be chosen to be 4 MB (AFAIU), so I was wondering whether you are referring to this effect, or to... > That is why we use a patched qemu version, which uses: >=20 > cluster_size =3D Max_Block_Size(source, target) ...this. The main problem with the stall I mentioned is that I think one of the main use cases of backup is having a fast source and a slow (off-site) target. In such cases, I suppose it becomes annoying if some guest writes (which were fast before the backup started) take a long time because the backup needs to copy quite a bit of data to off-site storage. (And blindly taking the source cluster size would mean that such things could happen if you use local qcow2 files with 2 MB clusters.) So I=E2=80=99d prefer decoupling the backup buffer size and the bitmap granularity, and then set the buffer size to maybe the MAX of source and target cluster sizes. But I don=E2=80=99t know when I can get around to do= that. And then probably also cap it at 4 MB or 8 MB, because that happens to be what you need, but I=E2=80=99d prefer for it not to use tons of memory. = (The mirror job uses 1 MB per request, for up to 16 parallel requests; and the backup copy-before-write implementation currently (on master) copies 1 MB at a time (per concurrent request), and the whole memory usage of backup is limited at 128 MB.) (OTOH, the minimum should probably be 1 MB.) Max --vHu5uIYB1yeVaGFkYDv8m47LIHQwfsTjW-- --Lpsgh7rRSgpOeBjYODGB10gn03uyUqGuW Content-Type: application/pgp-signature; name="signature.asc" Content-Description: OpenPGP digital signature Content-Disposition: attachment; filename="signature.asc" -----BEGIN PGP SIGNATURE----- iQEzBAEBCAAdFiEEkb62CjDbPohX0Rgp9AfbAGHVz0AFAl3C0AMACgkQ9AfbAGHV z0BoCAgAt6CZ96vbsSMNlGzzZiOVbQpnMeGCED91gpsXv2+lDPwe+tMGZOvFT3Se 3Iipp0OpCleDwuyfuOIj/KB0plQUb1oqLt8yOQxdAY13aX0NFZosiB4Mj2mHquyK i6S2NYZQb0dWVZxGs1AolUY6Hu53QZTP9qAaDXnLsw13yNqbHffC7nFKRB0sIL7C eBRsL4iL3B2Py25J/jLJvP+rkvi3iBaj7/Fh2BSneJ6cbUrRHSL7tNVZ4AnDNKX7 K59JPLxZvtu3nhGBpjrUVo0MkP0vv9RuAecuAbEM2EVoICO12K9hgdpRv5ujlJLL dy5Amk8VOkfnsuL6pam4UMK1bgMTWA== =Xkrc -----END PGP SIGNATURE----- --Lpsgh7rRSgpOeBjYODGB10gn03uyUqGuW--