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Wed, 5 Feb 2020 17:58:24 +0000 (UTC) Subject: Re: [PATCH 00/17] Improve qcow2 all-zero detection To: Vladimir Sementsov-Ogievskiy , Eric Blake , qemu-devel@nongnu.org References: <20200131174436.2961874-1-eblake@redhat.com> <3bad82d4-4d60-4341-d87e-af37e1dd680e@virtuozzo.com> <99576c66-00d2-14a3-5f1f-6d7cebfa6553@virtuozzo.com> <8b650616-ff80-c430-5d4e-6c9311bec2f6@redhat.com> <5494fd19-3dbe-8878-628b-20b3c0a0c6cd@virtuozzo.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: <51829e78-30d7-f0c6-f191-fc12c8cb37f4@redhat.com> Date: Wed, 5 Feb 2020 18:58:22 +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: X-Scanned-By: MIMEDefang 2.79 on 10.5.11.15 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="mXDqnAuaIs18jVZGAkwXmVYEWTlaajsAj" X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] [fuzzy] X-Received-From: 207.211.31.81 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: david.edmondson@oracle.com, 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) --mXDqnAuaIs18jVZGAkwXmVYEWTlaajsAj Content-Type: multipart/mixed; boundary="39a8O5WaOn0g0JTQzczNrv2DjtoiIDPZJ" --39a8O5WaOn0g0JTQzczNrv2DjtoiIDPZJ Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: quoted-printable On 05.02.20 16:14, Vladimir Sementsov-Ogievskiy wrote: > 05.02.2020 17:47, Vladimir Sementsov-Ogievskiy wrote: >> 05.02.2020 17:26, Eric Blake wrote: >>> On 2/5/20 3:25 AM, Vladimir Sementsov-Ogievskiy wrote: >>> >>>>> 3. For qcow2 >>>>> Hmm. Here, as I understand, than main case is freshly created qcow2, >>>>> which is fully-unallocated. To understand that it is empty, we >>>>> need only to check all L1 entries. And for empty L1 table it is fast. >>>>> So we don't need any qcow2 format improvement to check it. >>>>> >>>> >>>> Ah yes, I forget about preallocated case. Hmm. For preallocated >>>> clusters, >>>> we have zero bits in L2 entries. And with them, we even don't need >>>> preallocated to be filled by zeros, as we never read them (but just >>>> return >>>> zeros on read).. >>> >>> Scanning all L2 entries is O(n), while an autoclear bit properly >>> maintained is O(1). >>> >>>> >>>> Then, may be we want similar flag for L1 entry (this will enable large >>>> fast write-zero). And may be we want flag which marks the whole image >>>> as read-zero (it's your flag). So, now I think, my previous idea >>>> of "all allocated is zero" is worse. As for fully-preallocated images >>>> we are sure that all clusters are allocated, and it is more native to >>>> have flags similar to ZERO bit in L2 entry. >>> >>> Right now, we don't have any L1 entry flags.=C2=A0 Adding one would >>> require adding an incompatible feature flag (if older qemu would >>> choke to see unexpected flags in an L1 entry), or at best an >>> autoclear feature flag (if the autoclear bit gets cleared because an >>> older qemu opened the image and couldn't maintain L1 entry flags >>> correctly, then newer qemu knows it cannot trust those L1 entry >>> flags).=C2=A0 But as soon as you are talking about adding a feature bit= , >>> then why add one that still requires O(n) traversal to check (true, >>> the 'n' in an O(n) traversal of L1 tables is much smaller than the >>> 'n' in an O(n) traversal of L2 tables), when you can instead just add >>> an O(1) autoclear bit that maintains all_zero status for the image as >>> a whole? >>> >> >> My suggestion about L1 entry flag is side thing, I understand >> difference between O(n) and O(1) :) Still additional L1 entry will >> help to make efficient large block-status and write-zero requests. >> >> And I agree that we need top level flag.. I just try to say, that it >> seems good to make it similar with existing L2 flag. But yes, it would >> be incomaptible change, as it marks all clusters as ZERO, and older >> Qemu can't understand it and may treat all clusters as unallocated. >> >=20 > Still, how long is this O(n) ? We load the whole L1 into memory anyway. > For example, 16Tb disk with 64K granularity, we'll have 32768 L1 > entries. Will we get sensible performance benefit with an extension? I > doubt in it now. And anyway, if we have an extension, we should fallback > to this O(n) if we don't have the flag set. (Sorry, it=E2=80=99s late and I haven=E2=80=99t followed this particular co= nversation too closely, but:) Keep in mind that the default metadata overlap protection mode causes all L1 entries to be scanned on each I/O write. So it can=E2=80=99t be tha= t bad. Max --39a8O5WaOn0g0JTQzczNrv2DjtoiIDPZJ-- --mXDqnAuaIs18jVZGAkwXmVYEWTlaajsAj Content-Type: application/pgp-signature; name="signature.asc" Content-Description: OpenPGP digital signature Content-Disposition: attachment; filename="signature.asc" -----BEGIN PGP SIGNATURE----- iQEzBAEBCAAdFiEEkb62CjDbPohX0Rgp9AfbAGHVz0AFAl47Aj4ACgkQ9AfbAGHV z0DRUwf7BVebl3dPdDzZsKvYI4B/0J6aeJa80AOHeX6r9tjO7ZJUxvHC2REy/qdo s52CVUliAF1B67MTLCHalxop3qsqXSn5mrw5zohty6H3OMhDLXFLA5a+KtVy8DJw GbxYDpiBpPCpGLlbkKHiX23nOYqQw3Y9qa/1hsWdREwkMg9jF1WKB6N6OAzzngZd xbt6IVasgfwD0W6AEEYdVSBWomJOkROLr1ueINKklhVvTJ+izhAscB96a3DLyoxJ e8SZAlp97CLdc/yods1hO+fyopJkp2GcC37yQaobcXrk9DEQzZueHyHw96bP83W4 obfNOsMauXUfTQd5ewTj6AsDCIi6tg== =8fxA -----END PGP SIGNATURE----- --mXDqnAuaIs18jVZGAkwXmVYEWTlaajsAj--