From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-io0-f176.google.com ([209.85.223.176]:35021 "EHLO mail-io0-f176.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752594AbbH1RL4 (ORCPT ); Fri, 28 Aug 2015 13:11:56 -0400 Received: by ioed140 with SMTP id d140so12602183ioe.2 for ; Fri, 28 Aug 2015 10:11:56 -0700 (PDT) Subject: Re: Understanding BTRFS storage To: Chris Murphy , Hugo Mills , George Duffield , Roman Mamedov , Btrfs BTRFS References: <20150826165024.05af044f@natsu> <55DDAB1C.90406@gmail.com> <20150828093502.GO23769@carfax.org.uk> From: Austin S Hemmelgarn Message-ID: <55E09640.7030103@gmail.com> Date: Fri, 28 Aug 2015 13:11:28 -0400 MIME-Version: 1.0 In-Reply-To: Content-Type: multipart/signed; protocol="application/pkcs7-signature"; micalg=sha-512; boundary="------------ms060509060804060301090307" Sender: linux-btrfs-owner@vger.kernel.org List-ID: This is a cryptographically signed message in MIME format. --------------ms060509060804060301090307 Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: quoted-printable On 2015-08-28 11:42, Chris Murphy wrote: > On Fri, Aug 28, 2015 at 3:35 AM, Hugo Mills wrote:= >> On Fri, Aug 28, 2015 at 10:50:12AM +0200, George Duffield wrote: >>> Running a traditional raid5 array of that size is statistically >>> guaranteed to fail in the event of a rebuild. >> >> Except that if it were, you wouldn't see anyone running RAID-5 >> arrays of that size and (considerably) larger. And successfully >> replacing devices in them. >> >> As I understand it, the calculations that lead to the conclusion >> you quote are based on the assumption that the bit error rate (BER) of= >> the drive is applied on all reads -- this is not the case. The BER is >> the error rate of the platter after the device has been left unread >> (and powered off) for some long period of time. (I've seen 5 years >> been quoted for that). > > I think the confusion comes from the Unrecovered Read Error (URE) or > "Non-recoverable read errors per bits read" in the drive spec sheet. > e.g. on a WDC Red this is written as "<1 in 10^14" but this gets > (wrongly) reinterpreted into an *expected* URE once every 12.5TB (not > TiB) read, which is of course complete utter bullshit. But it gets > repeated all the time. > > It's as if symbols have no meaning, and < is some sort of arrow, or > someone got bored and just didn't want to use a space. That symbol > makes the URE value a maximum for what is ostensibly a scientific > sample of drives. We have no idea what the minimum is, we don't even > know the mean, and it's not in the manufacturer's best interest to do > that. The mean between consumer SATA and enterprise SAS may not be all > that different, while the maximum is two orders magnitude better for > enterprise SAS so it makes sense to try to upsell us with that > promise. > That probably is the case, the truly sad thing is that there are so many = engineers (read as 'people who are supposed to actually pay attention to = the specs') who do this on a regular basis. --------------ms060509060804060301090307 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 hvcNAQkDMQsGCSqGSIb3DQEHATAcBgkqhkiG9w0BCQUxDxcNMTUwODI4MTcxMTI4WjBPBgkq hkiG9w0BCQQxQgRAT7tC/yT2Zdcre0ZjYQY3Ur16yFYF2hSpBlVACFFuyjC4zmvZTYYVy3FP R6CneMnxxr6xwfJCuQ1JoBUPU5uUBjBsBgkqhkiG9w0BCQ8xXzBdMAsGCWCGSAFlAwQBKjAL BglghkgBZQMEAQIwCgYIKoZIhvcNAwcwDgYIKoZIhvcNAwICAgCAMA0GCCqGSIb3DQMCAgFA MAcGBSsOAwIHMA0GCCqGSIb3DQMCAgEoMIGRBgkrBgEEAYI3EAQxgYMwgYAweTEQMA4GA1UE ChMHUm9vdCBDQTEeMBwGA1UECxMVaHR0cDovL3d3dy5jYWNlcnQub3JnMSIwIAYDVQQDExlD QSBDZXJ0IFNpZ25pbmcgQXV0aG9yaXR5MSEwHwYJKoZIhvcNAQkBFhJzdXBwb3J0QGNhY2Vy dC5vcmcCAxBuVTCBkwYLKoZIhvcNAQkQAgsxgYOggYAweTEQMA4GA1UEChMHUm9vdCBDQTEe MBwGA1UECxMVaHR0cDovL3d3dy5jYWNlcnQub3JnMSIwIAYDVQQDExlDQSBDZXJ0IFNpZ25p bmcgQXV0aG9yaXR5MSEwHwYJKoZIhvcNAQkBFhJzdXBwb3J0QGNhY2VydC5vcmcCAxBuVTAN BgkqhkiG9w0BAQEFAASCAgBKQM+hXcZbBMaUbp3pccIIzOFn87Vtt1DEoqOrIEgXiDvH+Oae r2jJ65/fGxNDh6p75PxH3qtFcoKmiCfB1fpUfx2TrcFPh6Efd7qxGe3njeQRmmgvymXH9+5y k+i2NOx2AorFwTMrAkTNA352D2sbSiv30RdC9C5O3I5yXmS5GBhQRjitrB6yzkOX3b3kZDMU jzoE8U1L4asVbPQ/Fx5/Bq65FY9bTeRFkxllSxMv0VWpaC2esrAmJOnODI+NvfYppdBlBXX1 4F134P/SdT/AKwNN5TW3v3ZvV2gTLvlqzz8rtUn4dpAkf9X3GLS8ixhM3sY3GepIVoqfpYSD q8+mCp+jLCq7/AxH8NY7Xo75u+SY/wRtwhHkDJiuz2/S/8Uv6ZyS4/ODuCE0sTt1yXHqdzLg cQtRtKrhL1mxILLXq13ao1pgZvawI3S2bJ3UhvA2rCLOAuURX7Qtkfqnfy/oyzSifLjC36G1 fgsv7giTfZnVnEMEb4D37e/OriLr4HrCHNl69egCFhQLPHgC722JA2k7GQvHCnopu80N+XBq 1TaY5p4s5lfRPGK83Hh3k/L9b9VV2PqA7KVRrAjMj8y+1nBi1XtDnkS7AJWiRQJF8ypOyX8f HvBvmU8RJFkBWtJPj9sFv6PiYFb/8tZ7/TvE7SyHWqPYW8HpEx3tUfD4EwAAAAAAAA== --------------ms060509060804060301090307--