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Tue, 29 Oct 2019 11:55:30 +0000 (UTC) Received: from dresden.str.redhat.com (ovpn-116-124.ams2.redhat.com [10.36.116.124]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 4810560BF1; Tue, 29 Oct 2019 11:55:24 +0000 (UTC) Subject: Re: [RFC 0/3] block/file-posix: Work around XFS bug To: Vladimir Sementsov-Ogievskiy , Kevin Wolf , Stefan Hajnoczi References: <20191025095849.25283-1-mreitz@redhat.com> <20191027123555.GN4472@stefanha-x1.localdomain> <20191028110408.GB3579@localhost.localdomain> <44565375-b051-e782-4988-c3e0b1745e37@virtuozzo.com> <9780d020-e573-866f-dce4-d99d73f1f5e8@redhat.com> <98a5da91-d018-13e1-cf5c-168d7348117b@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: Date: Tue, 29 Oct 2019 12:55:22 +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: <98a5da91-d018-13e1-cf5c-168d7348117b@virtuozzo.com> 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="wm9jSvAwppve42Vap73ioP099imDNtR1q" 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: Anton Nefedov , Alberto Garcia , "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) --wm9jSvAwppve42Vap73ioP099imDNtR1q Content-Type: multipart/mixed; boundary="hmIPzFrhguELz5dAB5IrxFA5LNJKEKzlz" --hmIPzFrhguELz5dAB5IrxFA5LNJKEKzlz Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: quoted-printable On 29.10.19 12:48, Vladimir Sementsov-Ogievskiy wrote: > 29.10.2019 11:50, Max Reitz wrote: >> On 28.10.19 12:25, Vladimir Sementsov-Ogievskiy wrote: >>> 28.10.2019 14:04, Kevin Wolf wrote: >>>> Am 27.10.2019 um 13:35 hat Stefan Hajnoczi geschrieben: >>>>> On Fri, Oct 25, 2019 at 11:58:46AM +0200, Max Reitz wrote: >> >> [...] >> >>>>>> (3) Drop handle_alloc_space(), i.e. revert c8bb23cbdbe32f. >>>>>> To my knowledge I=E2=80=99m the only one who has provided any = benchmarks for >>>>>> this commit, and even then I was a bit skeptical because it pe= rforms >>>>>> well in some cases and bad in others. I concluded that it=E2= =80=99s >>>>>> probably worth it because the =E2=80=9Csome cases=E2=80=9D are= more likely to occur. >>>>>> >>>>>> Now we have this problem of corruption here (granted due to a = bug in >>>>>> the XFS driver), and another report of massively degraded >>>>>> performance on ppc64 >>>>>> (https://bugzilla.redhat.com/show_bug.cgi?id=3D1745823 =E2=80= =93 sorry, a >>>>>> private BZ; I hate that :-/ The report is about 40 % worse >>>>>> performance for an in-guest fio write benchmark.) >>>>>> >>>>>> So I have to ask the question about what the justification for >>>>>> keeping c8bb23cbdbe32f is. How much does performance increase= with >>>>>> it actually? (On non-(ppc64+XFS) machines, obviously) >>>>>> >>>>>> Advantages: >>>>>> + Trivial >>>>>> + No layering violations >>>>>> + We wouldn=E2=80=99t need to keep track of whether the kernel= bug has been >>>>>> fixed or not >>>>>> + Fixes the ppc64+XFS performance problem >>>>>> >>>>>> Disadvantages: >>>>>> - Reverts cluster allocation performance to pre-c8bb23cbdbe32f >>>>>> levels, whatever that means >>>>> >>>>> My favorite because it is clean and simple, but Vladimir has a valid >>>>> use-case for requiring this performance optimization so reverting isn= 't >>>>> an option. >>>> >>>> Vladimir also said that qcow2 subclusters would probably also solve hi= s >>>> problem, so maybe reverting and applying the subcluster patches instea= d >>>> is a possible solution, too? >>> >>> I'm not sure about ssd case, it may need write-zero optimization anyway= . >> >> What exactly do you need? Do you actually need to write zeroes (e.g. >> because you=E2=80=99re storing images on block devices) or would it be >> sufficient to just drop the COW areas when bdrv_has_zero_init() and >> bdrv_has_zero_init_truncate() are true? >=20 > Hmm, what do you mean? We need to zero COW areas. So, original way is to > write real zeroes, optimized way is fallocate.. What do you mean by drop? > Mark sublusters as zeroes by metadata? Why do you need to zero COW areas? For normal files, any data will read as zero if you didn=E2=80=99t write anything there. > But still we'll have COW areas in subcluster, and we'll need to directly = zero > them.. And fallocate will most probably be faster on ssd ext4 case.. >=20 >> >> I=E2=80=99m asking because Dave Chinner said >> (https://bugzilla.redhat.com/show_bug.cgi?id=3D1765547#c7) that >> fallocate() is always slow at least with aio=3Dnative because it needs t= o >> wait for all concurrent AIO writes to finish, and so it causes the AIO >> pipeline to stall. >> >> (He suggested using XFS extent size hints to get the same effect as >> write-zeroes for free, basically, but I don=E2=80=99t know whether that= =E2=80=99s really >> useful to us; as unallocated areas on XFS read back as zero anyway.) >> >>>> We already have some cases where the existing handle_alloc_space() >>>> causes performance to actually become worse, and serialising requests = as >>>> a workaround isn't going to make performance any better. So even on >>>> these grounds, keeping commit c8bb23cbdbe32f is questionable. >>>> >>> >>> Can keeping handle_alloc_space under some config option be an option? >> >> Hm. A config option is weird if you=E2=80=99re the only one who=E2=80= =99s going to >> enable it. But other than that I don=E2=80=99t have anything against it= . >> >=20 > It's just a bit easier for us to maintain config option, than out-of-tree= patch. > On the other hand, it's not a real problem to maintain this one patch in = separate. > It may return again to the tree, when XFS bug fixed. We=E2=80=99ll still have the problem that fallocate() must stall aio=3Dnati= ve requests. Max --hmIPzFrhguELz5dAB5IrxFA5LNJKEKzlz-- --wm9jSvAwppve42Vap73ioP099imDNtR1q Content-Type: application/pgp-signature; name="signature.asc" Content-Description: OpenPGP digital signature Content-Disposition: attachment; filename="signature.asc" -----BEGIN PGP SIGNATURE----- iQEzBAEBCAAdFiEEkb62CjDbPohX0Rgp9AfbAGHVz0AFAl24KKoACgkQ9AfbAGHV z0AjVAgAtgCz1izMjIe+aXu0LYvESyNsQn3Gmm/U32tqsT+IwbFrjzUxaqBrx2RZ 7BxHZAWPxWK+ppLc0eXrOLk3XSkmX6URXUGXsaXBrZEocDtbuRwq0HdPk6LVyJXy ardi3vK+gR52GRg5Vd7A8HzENkt1VxPqby5EInHJuk+GXxM19L18jmdNTYf0qhQf chY1xIuF6TMadkhSm/6wZrajNzblzXh4raEZXHyol3cvNw6E79LEfLjgQpJYcxBH dqazEI/D7lhuQ5AUCPGRmlhX1XihiGorXBazz4NYMk/KururMg0nq6EzRTYnhYOV 6hwrGx33THXzlXJs7GuIBFRJnVJ6oQ== =aawm -----END PGP SIGNATURE----- --wm9jSvAwppve42Vap73ioP099imDNtR1q--