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Fri, 3 Jul 2020 07:23:01 +0000 (UTC) Subject: Re: [PATCH v9 30/34] qcow2: Add prealloc field to QCowL2Meta To: Alberto Garcia , Eric Blake , qemu-devel@nongnu.org References: <0dd88b3b4d3e90b28267267d7686cf5350d9dea0.1593342067.git.berto@igalia.com> <687f5402-2316-e06a-2fd1-37375a4fa119@redhat.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: <1c121afb-bf65-5879-f9de-2edcaabb11f4@redhat.com> Date: Fri, 3 Jul 2020 09:22:59 +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: 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="EmTAO6k8dAzKIV2fEb4eHv6Tiaug3YmBM" Received-SPF: pass client-ip=207.211.31.120; envelope-from=mreitz@redhat.com; helo=us-smtp-1.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/07/03 02:53:25 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) --EmTAO6k8dAzKIV2fEb4eHv6Tiaug3YmBM Content-Type: multipart/mixed; boundary="VdarrO38aYCofpw1VDWFt1uu6a54avhGL" --VdarrO38aYCofpw1VDWFt1uu6a54avhGL Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: quoted-printable On 03.07.20 01:05, Alberto Garcia wrote: > On Thu 02 Jul 2020 05:09:47 PM CEST, Max Reitz wrote: >>> Without a backing file, there is no read required - writing to an >>> unallocated subcluster within a preallocated cluster merely has to >>> provide zeros to the rest of the write.=C2=A0 And depending on whether = we >>> can intelligently guarantee that the underlying protocol already >>> reads as zeroes when preallocated, we even have an optimization where >>> even that is not necessary.=C2=A0 We can still lump it in the "COW" >>> terminology, in that our write is more complex than merely writing in >>> place, but it isn't a true copy-on-write operation as there is >>> nothing to be copied. >> >> The term =E2=80=9CCOW=E2=80=9D specifically in the qcow2 driver also ref= ers to having >> to write zeroes to an area that isn=E2=80=99t written to by the guest as= part >> of the process of having to allocate a (sub)cluster. >=20 > The question is valid: if the space for the clusters is allocated but > the subclusters are not marked as such then any partial write request > will need to fill the rest with zeroes (in practice handle_alloc_space() > can do that efficiently but that's another question). >=20 > If there is a backing file then there's no other alternative because we > do need to copy the data from the backing file. >=20 > If there is no backing file perhaps we could allocate all subclusters as > well. I suppose we can detect that scenario at that point in the code (I > haven't checked) and I don't know what would happen if one later > attaches a backing file on runtime using the command-line options. >=20 > But what I would argue is that I don't see the benefit of using extended > L2 entries on an preallocated image with no backing file: other than > having twice as much L2 metadata what would be the use? The point of > subclusters is that they make allocation more efficient, but if the > image is already fully allocated then they give you nothing. That=E2=80=99s true. I didn=E2=80=99t think about it this way. Then indeed it doesn=E2=80=99t make sense to potentially break cases of lat= er adding a backing file: Reviewed-by: Max Reitz --VdarrO38aYCofpw1VDWFt1uu6a54avhGL-- --EmTAO6k8dAzKIV2fEb4eHv6Tiaug3YmBM Content-Type: application/pgp-signature; name="signature.asc" Content-Description: OpenPGP digital signature Content-Disposition: attachment; filename="signature.asc" -----BEGIN PGP SIGNATURE----- iQEzBAEBCAAdFiEEkb62CjDbPohX0Rgp9AfbAGHVz0AFAl7+3NMACgkQ9AfbAGHV z0C98wf/VoqRpdCDtr0WWPxiAHJS9aYm5Od/PseVlE3LLEQH9yUoYpThElyeuOFW 7O/8kes9445vDEL4WEXW96mDwRcVbe666M1dIdgNEeRTSj2QBCPubM13hdAKrrWl tts5gK6EM4QhZznWynPRLbpDwHi7e7De6hKRC0U96xnOjmYRjTNBK8R0zOmK/kbK DRH93Fdh0J3KgqHm94OLe6zVlyJQNbVwcc3pdzXeogiYcRb1qfT3A9/D7Nt2R2u4 HPg/p+x99ZI07dbjmUMhcJXxf4wOH0pPvOJ3tY0Gbe/0txDb0kkI+rF1CV/cPPfO mhBStSSXhARbYLF1uFz9cn5ERIk3Ww== =sdsI -----END PGP SIGNATURE----- --EmTAO6k8dAzKIV2fEb4eHv6Tiaug3YmBM--