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Wed, 6 Nov 2019 11:37:38 +0000 (UTC) Subject: Re: backup_calculate_cluster_size does not consider source From: Max Reitz 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> 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: <6684852e-da7d-13b2-f226-1c0074e4ab3b@redhat.com> Date: Wed, 6 Nov 2019 12:37:36 +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: 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="kYUh49cFw1nkL8MkyaWjtX82FJBlEDIpz" 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) --kYUh49cFw1nkL8MkyaWjtX82FJBlEDIpz Content-Type: multipart/mixed; boundary="PUYwGzJeDi0xP2akkEGbdkE1m4fcipuxd" --PUYwGzJeDi0xP2akkEGbdkE1m4fcipuxd Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: quoted-printable On 06.11.19 12:22, Max Reitz wrote: > On 06.11.19 12:18, Dietmar Maurer wrote: >>> And if it issues a smaller request, there is no way for a guest device >>> to tell it =E2=80=9COK, here=E2=80=99s your data, but note we have a wh= ole 4 MB chunk >>> around it, maybe you=E2=80=99d like to take that as well...?=E2=80=9D >>> >>> I understand wanting to increase the backup buffer size, but I don=E2= =80=99t >>> quite understand why we=E2=80=99d want it to increase to the source clu= ster size >>> when the guest also has no idea what the source cluster size is. >> >> Because it is more efficent. >=20 > For rbd. 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 there= =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). 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 causes = 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] Hm. OTOH, we have the same problem already with the target=E2=80=99s clust= er size, which can of course be 4 MB as well. But I can imagine it to actually be important for the target, because otherwise there might be read-modify-write cycles. But for the source, I still don=E2=80=99t quite understand why rbd has such= a problem with small read requests. I don=E2=80=99t doubt that it has (as yo= u explained), but again, how is it then even possible to use rbd as the backend for a guest that has no idea of this requirement? Does Linux really prefill the page cache with 4 MB of data for each read? Max [1] I suppose what we could do is decouple the copy buffer size from the bitmap granularity, but that would be more work than just a MAX() in backup_calculate_cluster_size(). --PUYwGzJeDi0xP2akkEGbdkE1m4fcipuxd-- --kYUh49cFw1nkL8MkyaWjtX82FJBlEDIpz Content-Type: application/pgp-signature; name="signature.asc" Content-Description: OpenPGP digital signature Content-Disposition: attachment; filename="signature.asc" -----BEGIN PGP SIGNATURE----- iQEzBAEBCAAdFiEEkb62CjDbPohX0Rgp9AfbAGHVz0AFAl3CsIEACgkQ9AfbAGHV z0AoSwf/d/Txdbo0pTIpvBTLg9otgjvgtyCSO5GviXlMGd6FjESajxRHeJXxD2aL r9lBHV7r/fv8CBp2RTUKcbRQYHMlVW6g6clg/QIwEB8KrZdwk0piw7ZP8gQsIonP UVdjjEL2YDe342NOGa9iZpRcMJDDdNgioPeGMp61IdodaOLCDozcOgHvAXRxiYYe bh0/y2Z1Q7pXdGK5PZPrQ9NO/ub4alLwuNAx09r+VQjzQ13Pls/z+x+SZJayBLym oND0yl5roH+TcFTQonFqIfsz/JTfVHBB+veg2O8kthYeZXGej8AgR4+t2ELmTUCi E4t6Zw6vWcrw9zu3AhB8btJFbBwUJw== =jFgU -----END PGP SIGNATURE----- --kYUh49cFw1nkL8MkyaWjtX82FJBlEDIpz--