From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-io0-f180.google.com ([209.85.223.180]:33091 "EHLO mail-io0-f180.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751068AbbIRNGQ (ORCPT ); Fri, 18 Sep 2015 09:06:16 -0400 Received: by iofh134 with SMTP id h134so55465856iof.0 for ; Fri, 18 Sep 2015 06:06:15 -0700 (PDT) Subject: Re: FYIO: A rant about btrfs To: Martin Steigerwald , Hugo Mills , Zia Nayamuth , linux-btrfs@vger.kernel.org References: <20150916144355.GA1285@invalid> <55F9BE3B.6070309@gmail.com> <20150916232930.GD28645@carfax.org.uk> <5147353.qIsJnOkqhM@merkaba> From: Austin S Hemmelgarn Message-ID: <55FC0C3A.9080702@gmail.com> Date: Fri, 18 Sep 2015 09:06:02 -0400 MIME-Version: 1.0 In-Reply-To: <5147353.qIsJnOkqhM@merkaba> Content-Type: multipart/signed; protocol="application/pkcs7-signature"; micalg=sha-512; boundary="------------ms080702060902050805090605" Sender: linux-btrfs-owner@vger.kernel.org List-ID: This is a cryptographically signed message in MIME format. --------------ms080702060902050805090605 Content-Type: text/plain; charset=windows-1252; format=flowed Content-Transfer-Encoding: quoted-printable On 2015-09-17 11:57, Martin Steigerwald wrote: > Am Mittwoch, 16. September 2015, 23:29:30 CEST schrieb Hugo Mills: >>> but even then having write-barriers >>> turned off is still not as safe as having them turned on. Most of >>> the time when I've tried testing with 'nobarrier' (not just on BTRFS >>> but on ext* as well), I had just as many issues with data loss when >>> the system crashed as when it (simlated via killing the virtual >>> machine) lost power. Both journaling and COW filesystems need to >>> ensure ordering of certain write operations to be able to maintain >>> consistency. For example, the new/updated data blocks need to be on >>> disk before the metadata is updated to point to them, otherwise you >>> database can end up corrupted. >> >> Indeed. The barriers are an ordering condition. The FS relies on >> (i.e. *requires*) that ordering condition, in order to be truly >> consistent. Running with "nobarrier" is a very strong signal that you >> really don't care about the data on the FS. >> >> This is not a case of me simply believing that because I've been >> using btrfs for so long that I've got used to the peculiarities. The >> first time I heard about the nobarrier option, something like 6 years >> ago when I was first using btrfs, I thought "that's got to be a really= >> silly idea". Any complex data structure, like a filesystem, is going >> to rely on some kind of ordering guarantees, somewhere in its >> structure. (The ordering might be strict, with a global clock, or >> barrier-based, or lattice-like, as for example a vector clock, but >> there's going to be _some_ concept of order). nobarrier allows the FS >> to ignore those guarantees, and even without knowing anything about >> the FS at all, doing so is a big red DANGER flag. > > Official recommendation for XFS differs from that: > > Q. Should barriers be enabled with storage which has a persistent wri= te > cache? > > Many hardware RAID have a persistent write cache which preserves it acr= oss > power failure, interface resets, system crashes, etc. Using write barri= ers in > this instance is not recommended and will in fact lower performance. > Therefore, it is recommended to turn off the barrier support and mount = the > filesystem with "nobarrier", assuming your RAID controller is infallibl= e and > not resetting randomly like some common ones do. But take care about th= e hard > disk write cache, which should be off. > > http://xfs.org/index.php/ > XFS_FAQ#Q._Should_barriers_be_enabled_with_storage_which_has_a_persiste= nt_write_cache. > 3F There's a difference there still, XFS isn't quite as dependent on=20 ordering as BTRFS is, and they're also giving some other strict=20 rquirements (hard-disk write-cache being off (which I see a rather large = number of people ignore), and a high-end RAID controller). --------------ms080702060902050805090605 Content-Type: application/pkcs7-signature; name="smime.p7s" Content-Transfer-Encoding: base64 Content-Disposition: attachment; filename="smime.p7s" Content-Description: S/MIME Cryptographic Signature MIAGCSqGSIb3DQEHAqCAMIACAQExDzANBglghkgBZQMEAgMFADCABgkqhkiG9w0BBwEAAKCC Brgwgga0MIIEnKADAgECAgMQblUwDQYJKoZIhvcNAQENBQAweTEQMA4GA1UEChMHUm9vdCBD QTEeMBwGA1UECxMVaHR0cDovL3d3dy5jYWNlcnQub3JnMSIwIAYDVQQDExlDQSBDZXJ0IFNp Z25pbmcgQXV0aG9yaXR5MSEwHwYJKoZIhvcNAQkBFhJzdXBwb3J0QGNhY2VydC5vcmcwHhcN MTUwMzI1MTkzNDM4WhcNMTUwOTIxMTkzNDM4WjBjMRgwFgYDVQQDEw9DQWNlcnQgV29UIFVz ZXIxIzAhBgkqhkiG9w0BCQEWFGFoZmVycm9pbjdAZ21haWwuY29tMSIwIAYJKoZIhvcNAQkB FhNhaGVtbWVsZ0BvaGlvZ3QuY29tMIICIjANBgkqhkiG9w0BAQEFAAOCAg8AMIICCgKCAgEA nQ/81tq0QBQi5w316VsVNfjg6kVVIMx760TuwA1MUaNQgQ3NyUl+UyFtjhpkNwwChjgAqfGd LIMTHAdObcwGfzO5uI2o1a8MHVQna8FRsU3QGouysIOGQlX8jFYXMKPEdnlt0GoQcd+BtESr pivbGWUEkPs1CwM6WOrs+09bAJP3qzKIr0VxervFrzrC5Dg9Rf18r9WXHElBuWHg4GYHNJ2V Ab8iKc10h44FnqxZK8RDN8ts/xX93i9bIBmHnFfyNRfiOUtNVeynJbf6kVtdHP+CRBkXCNRZ qyQT7gbTGD24P92PS2UTmDfplSBcWcTn65o3xWfesbf02jF6PL3BCrVnDRI4RgYxG3zFBJuG qvMoEODLhHKSXPAyQhwZINigZNdw5G1NqjXqUw+lIqdQvoPijK9J3eijiakh9u2bjWOMaleI SMRR6XsdM2O5qun1dqOrCgRkM0XSNtBQ2JjY7CycIx+qifJWsRaYWZz0aQU4ZrtAI7gVhO9h pyNaAGjvm7PdjEBiXq57e4QcgpwzvNlv8pG1c/hnt0msfDWNJtl3b6elhQ2Pz4w/QnWifZ8E BrFEmjeeJa2dqjE3giPVWrsH+lOvQQONsYJOuVb8b0zao4vrWeGmW2q2e3pdv0Axzm/60cJQ haZUv8+JdX9ZzqxOm5w5eUQSclt84u+D+hsCAwEAAaOCAVkwggFVMAwGA1UdEwEB/wQCMAAw VgYJYIZIAYb4QgENBEkWR1RvIGdldCB5b3VyIG93biBjZXJ0aWZpY2F0ZSBmb3IgRlJFRSBo ZWFkIG92ZXIgdG8gaHR0cDovL3d3dy5DQWNlcnQub3JnMA4GA1UdDwEB/wQEAwIDqDBABgNV HSUEOTA3BggrBgEFBQcDBAYIKwYBBQUHAwIGCisGAQQBgjcKAwQGCisGAQQBgjcKAwMGCWCG SAGG+EIEATAyBggrBgEFBQcBAQQmMCQwIgYIKwYBBQUHMAGGFmh0dHA6Ly9vY3NwLmNhY2Vy dC5vcmcwMQYDVR0fBCowKDAmoCSgIoYgaHR0cDovL2NybC5jYWNlcnQub3JnL3Jldm9rZS5j cmwwNAYDVR0RBC0wK4EUYWhmZXJyb2luN0BnbWFpbC5jb22BE2FoZW1tZWxnQG9oaW9ndC5j b20wDQYJKoZIhvcNAQENBQADggIBABr5e8W+NiTER+Q/7wiA2LxWN3UdhT3eZJjqqSlP370P KL5iWqeTfxQ67Ai/mHbJcT2PgAJ+/D2Ji+aRR03UWnU/vtOwzyDLUMstqnfl0Zs+sz/CJe7x nBA5jlpjC2DKuMVfbPze7eySaen7XSGFHKE1QoVIIpQ2kVjC4nbbJQnUbAVX1Iz29WxeVGt9 XYigz3tDPf3tglN+q23E7YjQl4abTIoM7i98yV1H9gfY8lFfKZ6jREB9+n6ie2EwS3Kat2mG tl2wBx4MfRnoSQSKsLKQ5oTwhWf0JqlFwpLfl374p0Njcykej9/jnWG8Ks1V/AXTHqI4eyIP Mf5yMZkPv7n7LS9WWKdG4Nd38iv4T2EiAaWsmgu+r81qL5CJu9AyA0SBS4ttKf6k3e63w2Mv N9R45vpQ3QhAhfWyFxFhZN95APe3YECDG3+XIRJpRYPEtHuIsOyzI70ajF93gg/BidvqKsmV MM2ccktDMfqwZXea6zey7F8Geu9R7BqjXmG2HlNuXu7e/xnHOgXf5D3wPmnRLlBhXL1Ch97a w2KjaupjpAHfFjv5kGnZXN87UvvlwzIZiKXwa3vTDwK+rrKn/sHPkfDZPSiyt/ZBIK6lX83P 34H/CzGg+Kx57rHYOIHGumIvpDa5vfWp8O0sGgawb1C2Aae4sTUVIWmIjVuGI062MYIE0TCC BM0CAQEwgYAweTEQMA4GA1UEChMHUm9vdCBDQTEeMBwGA1UECxMVaHR0cDovL3d3dy5jYWNl cnQub3JnMSIwIAYDVQQDExlDQSBDZXJ0IFNpZ25pbmcgQXV0aG9yaXR5MSEwHwYJKoZIhvcN AQkBFhJzdXBwb3J0QGNhY2VydC5vcmcCAxBuVTANBglghkgBZQMEAgMFAKCCAiEwGAYJKoZI hvcNAQkDMQsGCSqGSIb3DQEHATAcBgkqhkiG9w0BCQUxDxcNMTUwOTE4MTMwNjAyWjBPBgkq hkiG9w0BCQQxQgRAD3/bY11MdoFkHzH+SFlH1EEPvfVtcOyxc+j/gdt/LtH2BzTdDwRYKjFr ZpBwAlsP32PkGf0P7TEZt/vygVWZtTBsBgkqhkiG9w0BCQ8xXzBdMAsGCWCGSAFlAwQBKjAL BglghkgBZQMEAQIwCgYIKoZIhvcNAwcwDgYIKoZIhvcNAwICAgCAMA0GCCqGSIb3DQMCAgFA MAcGBSsOAwIHMA0GCCqGSIb3DQMCAgEoMIGRBgkrBgEEAYI3EAQxgYMwgYAweTEQMA4GA1UE ChMHUm9vdCBDQTEeMBwGA1UECxMVaHR0cDovL3d3dy5jYWNlcnQub3JnMSIwIAYDVQQDExlD QSBDZXJ0IFNpZ25pbmcgQXV0aG9yaXR5MSEwHwYJKoZIhvcNAQkBFhJzdXBwb3J0QGNhY2Vy dC5vcmcCAxBuVTCBkwYLKoZIhvcNAQkQAgsxgYOggYAweTEQMA4GA1UEChMHUm9vdCBDQTEe MBwGA1UECxMVaHR0cDovL3d3dy5jYWNlcnQub3JnMSIwIAYDVQQDExlDQSBDZXJ0IFNpZ25p bmcgQXV0aG9yaXR5MSEwHwYJKoZIhvcNAQkBFhJzdXBwb3J0QGNhY2VydC5vcmcCAxBuVTAN BgkqhkiG9w0BAQEFAASCAgAUVteoSmvCSISqw3+b3WpMtxGjtGSpuNNVchQv72HAbf8AGVYU ku9cbZ1WI1PtQpKje67UpYHD5Q1LKN+0WVTiA+VnLZ51PTsQshaVtLvCjfVZE5E81r0IYH2P wbFHzT3WOFGAatvLe9IkD1uoJQHv0ELIfQ/JjmxdbUqSn+WnkVx07onEkvVI3/5wMkHW0NqI joeOxKdskQzItNNCX7GhIwEMMIM4miS7LYxlY0zYUA8AeOFBPP2Cd2oRGY4ipCr72X9bfVP4 jL164VUw1Yb8nlFNfH0p4CovSloim713yr9V3HpeudgdnQiUt1QIfjLN9bO+XhXrjyfP6k/T llOz10ltMvZIZap/u02JtNrAblrArHXwouXGlBDqtiH7Y8MoqZVhbTMEbfwIT6rznjO1AkkQ LDbblV/qbJqmJzmavYZaCPgJzytcdETXE5pAEKjAqQ4mg1gzQ30EJlp7NHFExXVr5bMzNSaM QeadrCeQmtCKC7C8XL1AcM+zFXsp1pcQMamUcxlnwK50xX8vUT5bspPKFjbvhlYKBLEGfpNa 5x9qQRMupRwdO4vvsiEa1Dagqeb7n9zR6MhHM+QOl/0hJ5xDmb24UTL63ds/L9f44/vMdgkD ccUya18A+67uOjm1SPi4kEBtSN9iuTJ/LcLqHDNd4G/cjiZ0D1psCH+k/AAAAAAAAA== --------------ms080702060902050805090605--