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Wed, 5 Feb 2020 17:56:00 +0000 (UTC) Received: from dresden.str.redhat.com (ovpn-117-184.ams2.redhat.com [10.36.117.184]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 185671001B08; Wed, 5 Feb 2020 17:55:49 +0000 (UTC) Subject: Re: [PATCH 09/17] block: Refactor bdrv_has_zero_init{,_truncate} To: Eric Blake , Vladimir Sementsov-Ogievskiy , qemu-devel@nongnu.org References: <20200131174436.2961874-1-eblake@redhat.com> <20200131174436.2961874-10-eblake@redhat.com> <339f0a60-1e4f-286c-6594-1153bf284082@virtuozzo.com> <62078ab7-b46f-cb70-ffb1-5e84c7e1bee5@redhat.com> <8debd8e0-52bd-a962-0877-53dd0e610f1d@virtuozzo.com> <945a62b8-6b50-70bf-bb7e-daacc00d5e43@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: <4f193e34-93bc-fd63-18cc-b9e0d9ee201d@redhat.com> Date: Wed, 5 Feb 2020 18:55:48 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.4.1 MIME-Version: 1.0 In-Reply-To: <945a62b8-6b50-70bf-bb7e-daacc00d5e43@redhat.com> X-Scanned-By: MIMEDefang 2.84 on 10.5.11.22 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="H2mN7ZUcEkEqJvXeA1wKUvd9iTKAV8EMI" 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: Kevin Wolf , Fam Zheng , "open list:Sheepdog" , qemu-block@nongnu.org, Jeff Cody , Stefan Weil , Peter Lieven , "Richard W.M. Jones" , Markus Armbruster , david.edmondson@oracle.com, Stefan Hajnoczi , "Denis V. Lunev" , Liu Yuan , Jason Dillaman 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) --H2mN7ZUcEkEqJvXeA1wKUvd9iTKAV8EMI Content-Type: multipart/mixed; boundary="j0DTE32KOkoZG7pmiNEH12YkjRRx63rNQ" --j0DTE32KOkoZG7pmiNEH12YkjRRx63rNQ Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: quoted-printable On 05.02.20 15:07, Eric Blake wrote: > On 2/5/20 1:51 AM, Vladimir Sementsov-Ogievskiy wrote: >=20 >>>>> +typedef enum { >>>>> +=C2=A0=C2=A0=C2=A0 /* >>>>> +=C2=A0=C2=A0=C2=A0=C2=A0 * bdrv_known_zeroes() should include this b= it if the contents of >>>>> +=C2=A0=C2=A0=C2=A0=C2=A0 * a freshly-created image with no backing f= ile reads as all >>>>> +=C2=A0=C2=A0=C2=A0=C2=A0 * zeroes without any additional effort.=C2= =A0 If .bdrv_co_truncate is >>>>> +=C2=A0=C2=A0=C2=A0=C2=A0 * set, then this must be clear if BDRV_ZERO= _TRUNCATE is clear. >>>> >>>> I understand that this is preexisting logic, but could I ask: why? >>>> What's wrong >>>> if driver can guarantee that created file is all-zero, but is not sure >>>> about >>>> file resizing? I agree that it's normal for these flags to have the >>>> same >>>> value, >>>> but what is the reason for this restriction?.. >>> >>> If areas added by truncation (or growth, rather) are always zero, then >>> the file can always be created with size 0 and grown from there.=C2=A0 = Thus, >>> images where truncation adds zeroed areas will generally always be zero >>> after creation. >> >> This means, that if truncation bit is set, than create bit should be >> set.. But >> here we say that if truncation is clear, than create bit must be clear. >=20 > Max, did we get the logic backwards? Or maybe my explanation was just wrong. Because nobody actually forces a driver to use truncate to ensure that an newly created file will be 0. Hm. And more importantly, you can=E2=80= =99t use truncate with PREALLOC_MODE_OFF when you want to create an image with preallocation. Let=E2=80=99s see. The offending commit message says: > No .bdrv_has_zero_init() implementation returns 1 if growing the file > would add non-zero areas (at least with PREALLOC_MODE_OFF), so using it > in lieu of this new function was always safe. >=20 > But on the other hand, it is possible that growing an image that is not > zero-initialized would still add a zero-initialized area, like when > using nonpreallocating truncation on a preallocated image. For callers > that care only about truncation, not about creation with potential > preallocation, this new function is useful. So I suppose the explanation is just the preallocation mode alone; has_zero_init() is for the image=E2=80=99s actual preallocation mode, where= as has_zero_init_truncate() is forced to PREALLOC_MODE_OFF. As such, the latter is less strict than the former. So the former cannot be true when the latter is false. >>>> So, the only possible combination of flags, when they differs, is >>>> create=3D0 and >>>> truncate=3D1.. How is it possible? >>> >>> For preallocated qcow2 images, it depends on the storage whether they >>> are actually 0 after creation.=C2=A0 Hence qcow2_has_zero_init() then d= efers >>> to bdrv_has_zero_init() of s->data_file->bs. >>> >>> But when you truncate them (with PREALLOC_MODE_OFF, as >>> BlockDriver.bdrv_has_zero_init_truncate()=E2=80=99s comment explains), = the new >>> area is always going to be 0, regardless of initial preallocation. >> >> ah yes, due to qcow2 zero clusters. >=20 > Hmm. Do we actually set the zero flag on unallocated clusters when > resizing a qcow2 image? No. They are just unallocated, i.e. zero. (Nodes with backing files never return true for bdrv_has_zero_init_truncate anyway). > That would be an O(n) operation (we have to > visit the L2 entry for each added cluster, even if only to set the zero > cluster bit).=C2=A0 Or do we instead just rely on the fact that qcow2 is > inherently sparse, and that when you resize the guest-visible size > without writing any new clusters, then it is only subsequent guest > access to those addresses that finally allocate clusters, making resize > O(1) (update the qcow2 metadata cluster, but not any L2 tables) while > still reading 0 from the new data.=C2=A0 To some extent, that's what the > allocation mode is supposed to control. >=20 > What about with external data images, where a resize in guest-visible > length requires a resize of the underlying data image?=C2=A0 There, we DO > have to worry about whether the data image resizes with zeroes (as in > the filesystem) or with random data (as in a block device). Well, partially: Namely, only with data_file_raw. Because otherwise the clusters are still unallocated and thus read as zero. So yes, then we do have to worry about that. With data_file_raw, we have an obligation to make the data file return the same data as the qcow2 file, so, um. I wonder whether we actually take any care of this yet. If you have some external data file without zero_init(_truncate), do get zeroes when reading from the qcow2 node, but non-zeroes when reading from the raw data file? That would be OK without data_file_raw, but not with it. I suppose I=E2=80=99ll have to tes= t it. Max --j0DTE32KOkoZG7pmiNEH12YkjRRx63rNQ-- --H2mN7ZUcEkEqJvXeA1wKUvd9iTKAV8EMI Content-Type: application/pgp-signature; name="signature.asc" Content-Description: OpenPGP digital signature Content-Disposition: attachment; filename="signature.asc" -----BEGIN PGP SIGNATURE----- iQEzBAEBCAAdFiEEkb62CjDbPohX0Rgp9AfbAGHVz0AFAl47AaQACgkQ9AfbAGHV z0CT2Qf8DAyTuPb4kUQexgQR6XLA3zHg/+FGVUtG4o640uwxmWcnOTqF16KE7Wlw oSJRZxsomvdJq6jg7CJxQ3PCQd2lGoTY+ZACpafE5ZaNNGtJfDLcHzbyfprmo2J7 jr177mMGbfXL84QFmMo0hhg032nooYQTAwZMswkWYiilBZ0s0GZ25QTydoEVYQ14 pyD7uc89pjGEvpnCG6lsTDuGZK+COSYr0VG9rxO9WRhwu43nOF1kliSH40LqV2FZ ianxddUm/wiqyG1TgmMjS+vptQMGWK5K4+N5GVzANc0SOyAzesFVOeReo0j6PYJL zYbQlRVRhB8UCQEfj/s7Rh3AHRXAGw== =qQDx -----END PGP SIGNATURE----- --H2mN7ZUcEkEqJvXeA1wKUvd9iTKAV8EMI--