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Fri, 12 Jul 2019 11:48:31 +0000 (UTC) To: Kevin Wolf References: <20190711195804.30703-1-mreitz@redhat.com> <20190711195804.30703-3-mreitz@redhat.com> <20190712094919.GC4514@dhcp-200-226.str.redhat.com> <20190712111720.GF4514@dhcp-200-226.str.redhat.com> From: Max Reitz Openpgp: preference=signencrypt 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: <78d1d91b-8a13-9b9b-a891-650c6b4d417b@redhat.com> Date: Fri, 12 Jul 2019 13:48:30 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.7.2 MIME-Version: 1.0 In-Reply-To: <20190712111720.GF4514@dhcp-200-226.str.redhat.com> Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="DQ105oUOcUuFFszzpfwr1n2USlarOM3dP" X-Scanned-By: MIMEDefang 2.84 on 10.5.11.23 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.45]); Fri, 12 Jul 2019 11:48:35 +0000 (UTC) X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] X-Received-From: 209.132.183.28 Subject: Re: [Qemu-devel] [RFC 2/5] block: Generic truncation fallback 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: 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) --DQ105oUOcUuFFszzpfwr1n2USlarOM3dP Content-Type: multipart/mixed; boundary="l9bhMtGjmJv9cuyZ1GOutFxCy3Q7wp5PW"; protected-headers="v1" From: Max Reitz To: Kevin Wolf Cc: qemu-block@nongnu.org, qemu-devel@nongnu.org, Eric Blake , Stefan Hajnoczi Message-ID: <78d1d91b-8a13-9b9b-a891-650c6b4d417b@redhat.com> Subject: Re: [RFC 2/5] block: Generic truncation fallback References: <20190711195804.30703-1-mreitz@redhat.com> <20190711195804.30703-3-mreitz@redhat.com> <20190712094919.GC4514@dhcp-200-226.str.redhat.com> <20190712111720.GF4514@dhcp-200-226.str.redhat.com> In-Reply-To: <20190712111720.GF4514@dhcp-200-226.str.redhat.com> --l9bhMtGjmJv9cuyZ1GOutFxCy3Q7wp5PW Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: quoted-printable On 12.07.19 13:17, Kevin Wolf wrote: > Am 12.07.2019 um 12:58 hat Max Reitz geschrieben: >> On 12.07.19 11:49, Kevin Wolf wrote: >>> Am 11.07.2019 um 21:58 hat Max Reitz geschrieben: >>>> If a protocol driver does not support truncation, we call fall back = to >>>> effectively not doing anything if the new size is less than the actu= al >>>> file size. This is what we have been doing for some host device dri= vers >>>> already. >>> >>> Specifically, we're doing it for drivers that access a fixed-size ima= ge, >>> i.e. block devices rather than regular files. We don't want to do thi= s >>> for drivers where the file size could be changed, but just didn't >>> implement it. >>> >>> So I would suggest calling the function more specifically something l= ike >>> bdrv_co_truncate_blockdev(), and not using it as an automatic fallbac= k >>> in bdrv_co_truncate(), but just make it the BlockDriver.bdrv_co_trunc= ate >>> implementation for those drivers where it makes sense. >> >> I was thinking about this, but the problem is that .bdrv_co_truncate()= >> does not get a BdrvChild, so an implementation for it cannot generical= ly >> zero the first sector (without bypassing the permission system, which >> would be wrong). >=20 > Hm, I see. The next best thing would then probably having a bool in > BlockDriver that enables the fallback. >=20 >> So the function pointer would actually need to be set to something lik= e >> (int (*)(BlockDriverState *, int64_t, PreallocMode, Error **))42ul, or= a >> dummy function that just aborts, and then bdrv_co_truncate() would >> recognize this magic constant. But that seemed so weird to me that I >> decided just not to do it, mostly because I was wondering what would b= e >> so bad about treating images whose size we cannot change because we >> haven=E2=80=99t implemented it exactly like fixed-size images. >> >> (Also, =E2=80=9Cfixed-size=E2=80=9D is up to interpretation. You can = change an LVM >> volume=E2=80=99s size. qemu doesn=E2=80=99t do it, obviously. But th= at is the reason >> for the warning qemu-img resize emits when it sees that the file size >> did not change.) >> >>> And of course, we only need these fake implementations because qemu-i= mg >>> (or .bdrv_co_create_opts) always wants to create the protocol level. = If >>> we could avoid this, then we wouldn't need any of this. >> >> It=E2=80=99s trivial to avoid this. I mean, patch 4 does exactly that= =2E >> >> So it isn=E2=80=99t about avoiding creating the protocol level, it=E2=80= =99s about >> avoiding the truncation there. But why would you do that? >=20 > Because we can't actually truncate there. We can only do the fake thing= > and claim we have truncated even though the size didn't change. You=E2=80=99re right. I actually didn=E2=80=99t realize that we have no = drivers that support truncation, but not image creation. Yes, then it=E2=80=99s stupid. I thought it was a reasonable thing to do for such drivers. So I suppose the best thing is to do what I hinted at in my reply to your reply to patch 3, which is to drop patches 2 and 3 and instead make the creation fallback work around blk_truncate() failures. Max > But actually, while avoiding the fake truncate outside of image creatio= n > would avoid confusing situations where the user requested image > shrinking, gets success, but nothing happened, it would also break > actual resizes where the volume is resized externally and block_resize > is only called to notify qemu to pick up the size change. >=20 > So after all, we probably do need to keep the hack in bdrv_co_truncate(= ) > instead of moving it to image creation only. >=20 > Kevin >=20 --l9bhMtGjmJv9cuyZ1GOutFxCy3Q7wp5PW-- --DQ105oUOcUuFFszzpfwr1n2USlarOM3dP Content-Type: application/pgp-signature; name="signature.asc" Content-Description: OpenPGP digital signature Content-Disposition: attachment; filename="signature.asc" -----BEGIN PGP SIGNATURE----- iQEzBAEBCAAdFiEEkb62CjDbPohX0Rgp9AfbAGHVz0AFAl0oc44ACgkQ9AfbAGHV z0Cf8Af7BTBwUbzD0i3Wbrw0gInoRHubbedLt3JFz6/4hFaudHfLtVeU/65i4Dv1 RkF6W/DDK+K/PWgtTVsrCxb9vXVQnb35Us113jc2PYweaVOuRWgy5mPjbTtll25V VgsooFF9u75EXc5EjNySnmg9IlvrxbTrqHUY5bFOehY5mVOrzVckEjcc/Fp6hMi8 Aju7OdPsUC8q/zZVSMrXMsQJp5pZ9E3U4ySyCDO7nARTZgc45GgCOA5VtEIDMs7z MM+nqEH9gJAgn6z+zlp1kbNrTbERt51jUAi2UwqjjpHJSo/ZHMRaHO/LC7VsakRq qbMqpEirNvchrR6tkURVXaQnsHgvLw== =Z44V -----END PGP SIGNATURE----- --DQ105oUOcUuFFszzpfwr1n2USlarOM3dP--