From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-io0-f181.google.com ([209.85.223.181]:34843 "EHLO mail-io0-f181.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752321AbbHGLjP (ORCPT ); Fri, 7 Aug 2015 07:39:15 -0400 Received: by iodd187 with SMTP id d187so108910693iod.2 for ; Fri, 07 Aug 2015 04:39:14 -0700 (PDT) Subject: Re: btrfs raid1 metadata, single data To: Mike Fleetwood , Sjoerd References: <55C47136.2080402@render-wahnsinn.de> <201508071918.02231.russell@coker.com.au> <1520215.AbHMhTRmNr@hoefnix> Cc: linux-btrfs From: Austin S Hemmelgarn Message-ID: <55C498D1.7030807@gmail.com> Date: Fri, 7 Aug 2015 07:38:57 -0400 MIME-Version: 1.0 In-Reply-To: Content-Type: multipart/signed; protocol="application/pkcs7-signature"; micalg=sha-512; boundary="------------ms030007080705010603020605" Sender: linux-btrfs-owner@vger.kernel.org List-ID: This is a cryptographically signed message in MIME format. --------------ms030007080705010603020605 Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: quoted-printable On 2015-08-07 06:40, Mike Fleetwood wrote: > On 7 August 2015 at 10:47, Sjoerd wrote: >> While we're at it: any idea why the default for SSD's is single for me= ta data >> as described on the wiki? >> (https://btrfs.wiki.kernel.org/index.php/Using_Btrfs_with_Multiple_Dev= ices#Filesystem_creation) >> >> I was looking for an answer why my SSD just had single metadata, while= I >> expected it to be DUP and stumbled on this wiki article. Can't find a = reason >> for why a SSD would be different? >> >> Cheers, >> Sjoerd > > I would assume that it is because some SSD drives controllers > deduplicate by default [1]. The developers probably think that when > it comes to your data the truth, no mater how ugly, is preferable to a > false sense of security. (Btrfs thinking it has 2 copies of metadata > when the SSD drive only actually has stored 1 copy). > > [1] How SSDs can hose your data > http://www.zdnet.com/article/how-ssds-can-hose-your-data/ > "Researchers found that at least 1 Sandforce SSD controller - the > SF1200 - does block-level deduplication by default. Which can be a > problem. > > Many file systems - NTFS, most Unix/Linux FSs, ZFS are some - write > critical metadata to multiple blocks in case one copy gets corrupted. > But what if, unbeknownst to you, your SSD de-duplicates that block, > leaving your file system with only 1 copy? " > And of course there's the counter-argument from the manufacturers that=20 do this: "But we use ECC, so only having one copy of the data is still safe!" which is obviously something from their marketing department and not the = people who actually understand how this works, most SSD's only do SECDED = (Single error correction, double error detection) ECC, which is very=20 much insufficient if it's MLC flash, as losing a single cell causes=20 multiple bits to go bad. The other reason, which applies to all SSD's, is that the oisk layout is = very different from how things are actually stored on the flash chips=20 themselves, and most firmware will group writes temporally, likely=20 causing both copies of the metadata to be put on the same erase block,=20 which in turn means that the duplication provides essentially zero=20 protection beyond having a single copy. --------------ms030007080705010603020605 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 hvcNAQkDMQsGCSqGSIb3DQEHATAcBgkqhkiG9w0BCQUxDxcNMTUwODA3MTEzODU3WjBPBgkq hkiG9w0BCQQxQgRAOy/Kip1rVTDB8Y0bEOEfm7HXdb0bMfmeoNwHfUFy/lWmHnISRuWodfAo xpjYaWMe9QSpPgcbjOZCezQp3lml+jBsBgkqhkiG9w0BCQ8xXzBdMAsGCWCGSAFlAwQBKjAL BglghkgBZQMEAQIwCgYIKoZIhvcNAwcwDgYIKoZIhvcNAwICAgCAMA0GCCqGSIb3DQMCAgFA MAcGBSsOAwIHMA0GCCqGSIb3DQMCAgEoMIGRBgkrBgEEAYI3EAQxgYMwgYAweTEQMA4GA1UE ChMHUm9vdCBDQTEeMBwGA1UECxMVaHR0cDovL3d3dy5jYWNlcnQub3JnMSIwIAYDVQQDExlD QSBDZXJ0IFNpZ25pbmcgQXV0aG9yaXR5MSEwHwYJKoZIhvcNAQkBFhJzdXBwb3J0QGNhY2Vy dC5vcmcCAxBuVTCBkwYLKoZIhvcNAQkQAgsxgYOggYAweTEQMA4GA1UEChMHUm9vdCBDQTEe MBwGA1UECxMVaHR0cDovL3d3dy5jYWNlcnQub3JnMSIwIAYDVQQDExlDQSBDZXJ0IFNpZ25p bmcgQXV0aG9yaXR5MSEwHwYJKoZIhvcNAQkBFhJzdXBwb3J0QGNhY2VydC5vcmcCAxBuVTAN BgkqhkiG9w0BAQEFAASCAgAyEXMHu1qdcC16lySflViSqMsXDZFAN6TLG33xmqQIFYSNA+7t lViN12+tQUea7NURUst5qlVUJip2sRMZ5FEoJZmjFh7oe6E/xBuEw+EhhRM0kK5lUKcRZYVb mO/qvTDQvw4+WyWSE6NizdyH0g5S4l3WOPVmCkaa6s2fs4ZL2iCJUvgNu5gqoZi4KFBICn1n W4rpuJmxMBFNXxPF39Nphrk2bCmA/TVvNcvyvm+l1Y/emGxG2uuvOJVu1LAHA9dJPTudCcIJ 4kXFjK3ObnXvw0TASRtjP0jkrijQxI9Md5aWFIxXdqpbQuhTlunCWsCsNo8vJ+PJygx4A3bT MFamVo3nZ1mDUxdZTypDqxgtxxU06j76VNQxF1aLD32VX2K3l03WVoayGduoCBzAzMaFYo4e vdz0m40BNFo6Ii8n3o94KmuRQbXU4TBRFl2/ZxbPlYvV6+7fVXxDju8dSRUGsHh9ht5Wjpb+ 0fbHB7OG+qke2oBgfLJlleOOEqIHHZxumf/FHw9fFyn93bXk7N6nTbQSzdG1nSF2cSWsBJC/ /aF9PRotRKLgPJqdZJgZ1qYVU9NadHn/G1lyDpinzmhezzGlnTgFHQR0sHJpCH5+AlMlvThM wxGroZlfv392soWIua9mRYVB+z+u42+wxCQYjr4zVQOgerNZdSm9ayschQAAAAAAAA== --------------ms030007080705010603020605--