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b=QykLsJCW59xnWeu63T8fQTHZNAFGNim/Rfix4bT/RFmfsGF3rabxNk2qMrLbqsUl5 xr1IrzWwDtbSDKxyf3kuwUidrojmY5ATAyu7qSbu/3PsWlX7A+3KL2IrgHRrpvvtMB qDwimZDEL9KfPEhgdy5ivh3Rj8EIhvAW8WfbLjQvh4WYEtYkQ8FFEL3bDl16ZUOMtq Xon0W4EZLikEygOj2uusBS7i+pq7UA2dGFXE/vR4I9zQDbpNBwq1jXjcWrkH8bXusy yNxtyqzr5GoynC1FC+sQ/8cbmsYeY+h8vLTIVkonCgAqtdC2uGPcjLSkao7bmO/zOY kkqHFJ9fZhv6g== Received: from [192.168.0.211] (novatech.home.cobb.me.uk [192.168.0.211]) by black.home.cobb.me.uk (Postfix) with ESMTPS id E147E5B973; Thu, 17 Oct 2019 19:23:40 +0100 (BST) Subject: Re: MD RAID 5/6 vs BTRFS RAID 5/6 To: Chris Murphy , Btrfs BTRFS Cc: Qu Wenruo References: From: Graham Cobb Openpgp: preference=signencrypt Autocrypt: addr=g.btrfs@cobb.uk.net; prefer-encrypt=mutual; keydata= mQINBFaetnIBEAC5cHHbXztbmZhxDof6rYh/Dd5otxJXZ1p7cjE2GN9hCH7gQDOq5EJNqF9c VtD9rIywYT1i3qpHWyWo0BIwkWvr1TyFd3CioBe7qfo/8QoeA9nnXVZL2gcorI85a2GVRepb kbE22X059P1Z1Cy7c29dc8uDEzAucCILyfrNdZ/9jOTDN9wyyHo4GgPnf9lW3bKqF+t//TSh SOOis2+xt60y2In/ls29tD3G2ANcyoKF98JYsTypKJJiX07rK3yKTQbfqvKlc1CPWOuXE2x8 DdI3wiWlKKeOswdA2JFHJnkRjfrX9AKQm9Nk5JcX47rLxnWMEwlBJbu5NKIW5CUs/5UYqs5s 0c6UZ3lVwinFVDPC/RO8ixVwDBa+HspoSDz1nJyaRvTv6FBQeiMISeF/iRKnjSJGlx3AzyET ZP8bbLnSOiUbXP8q69i2epnhuap7jCcO38HA6qr+GSc7rpl042mZw2k0bojfv6o0DBsS/AWC DPFExfDI63On6lUKgf6E9vD3hvr+y7FfWdYWxauonYI8/i86KdWB8yaYMTNWM/+FAKfbKRCP dMOMnw7bTbUJMxN51GknnutQlB3aDTz4ze/OUAsAOvXEdlDYAj6JqFNdZW3k9v/QuQifTslR JkqVal4+I1SUxj8OJwQWOv/cAjCKJLr5g6UfUIH6rKVAWjEx+wARAQABtDNHcmFoYW0gQ29i YiAoUGVyc29uYWwgYWRkcmVzcykgPGdyYWhhbUBjb2JiLnVrLm5ldD6JAlEEEwECADsCGwEG CwkIBwMCBhUIAgkKCwQWAgMBAh4BAheAAhkBBQJWnr9UFRhoa3A6Ly9rZXlzLmdudXBnLm5l dAAKCRBv35GGXfm3Tte8D/45+/dnVdvzPsKgnrdoXpmvhImGaSctn9bhAKvng7EkrQjgV3cf C9GMgK0vEJu+4f/sqWA7hPKUq/jW5vRETcvqEp7v7z+56kqq5LUQE5+slsEb/A4lMP4ppwd+ TPwwDrtVlKNqbKJOM0kPkpj7GRy3xeOYh9D7DtFj2vlmaAy6XvKav/UUU4PoUdeCRyZCRfl0 Wi8pQBh0ngQWfW/VqI7VsG3Qov5Xt7cTzLuP/PhvzM2c5ltZzEzvz7S/jbB1+pnV9P7WLMYd EjhCYzJweCgXyQHCaAWGiHvBOpmxjbHXwX/6xTOJA5CGecDeIDjiK3le7ubFwQAfCgnmnzEj pDG+3wq7co7SbtGLVM3hBsYs27M04Oi2aIDUN1RSb0vsB6c07ECT52cggIZSOCvntl6n+uMl p0WDrl1i0mJUbztQtDzGxM7nw+4pJPV4iX1jJYbWutBwvC+7F1n2F6Niu/Y3ew9a3ixV2+T6 aHWkw7/VQvXGnLHfcFbIbzNoAvI6RNnuEqoCnZHxplEr7LuxLR41Z/XAuCkvK41N/SOI9zzT GLgUyQVOksdbPaxTgBfah9QlC9eXOKYdw826rGXQsvG7h67nqi67bp1I5dMgbM/+2quY9xk0 hkWSBKFP7bXYu4kjXZUaYsoRFEfL0gB53eF21777/rR87dEhptCnaoXeqbkBDQRWnrnDAQgA 0fRG36Ul3Y+iFs82JPBHDpFJjS/wDK+1j7WIoy0nYAiciAtfpXB6hV+fWurdjmXM4Jr8x73S xHzmf9yhZSTn3nc5GaK/jjwy3eUdoXu9jQnBIIY68VbgGaPdtD600QtfWt2zf2JC+3CMIwQ2 fK6joG43sM1nXiaBBHrr0IadSlas1zbinfMGVYAd3efUxlIUPpUK+B1JA12ZCD2PCTdTmVDe DPEsYZKuwC8KJt60MjK9zITqKsf21StwFe9Ak1lqX2DmJI4F12FQvS/E3UGdrAFAj+3HGibR yfzoT+w9UN2tHm/txFlPuhGU/LosXYCxisgNnF/R4zqkTC1/ao7/PQARAQABiQIlBBgBAgAP BQJWnrnDAhsMBQkJZgGAAAoJEG/fkYZd+bdO9b4P/0y3ADmZkbtme4+Bdp68uisDzfI4c/qo XSLTxY122QRVNXxn51yRRTzykHtv7/Zd/dUD5zvwj2xXBt9wk4V060wtqh3lD6DE5mQkCVar eAfHoygGMG+/mJDUIZD56m5aXN5Xiq77SwTeqJnzc/lYAyZXnTAWfAecVSdLQcKH21p/0AxW GU9+IpIjt8XUEGThPNsCOcdemC5u0I1ZeVRXAysBj2ymH0L3EW9B6a0airCmJ3Yctm0maqy+ 2MQ0Q6Jw8DWXbwynmnmzLlLEaN8wwAPo5cb3vcNM3BTcWMaEUHRlg82VR2O+RYpbXAuPOkNo 6K8mxta3BoZt3zYGwtqc/cpVIHpky+e38/5yEXxzBNn8Rn1xD6pHszYylRP4PfolcgMgi0Ny 72g40029WqQ6B7bogswoiJ0h3XTX7ipMtuVIVlf+K7r6ca/pX2R9B/fWNSFqaP4v0qBpyJdJ LO/FP87yHpEDbbKQKW6Guf6/TKJ7iaG3DDpE7CNCNLfFG/skhrh5Ut4zrG9SjA+0oDkfZ4dI B8+QpH3mP9PxkydnxGiGQxvLxI5Q+vQa+1qA5TcCM9SlVLVGelR2+Wj2In+t2GgigTV3PJS4 tMlN++mrgpjfq4DMYv1AzIBi6/bSR6QGKPYYOOjbk+8Sfao0fmjQeOhj1tAHZuI4hoQbowR+ myxb Message-ID: Date: Thu, 17 Oct 2019 19:23:40 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.9.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-GB Content-Transfer-Encoding: 7bit Sender: linux-btrfs-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-btrfs@vger.kernel.org On 17/10/2019 16:57, Chris Murphy wrote: > On Wed, Oct 16, 2019 at 10:07 PM Jon Ander MB wrote: >> >> It would be interesting to know the pros and cons of this setup that >> you are suggesting vs zfs. >> +zfs detects and corrects bitrot ( >> http://www.zfsnas.com/2015/05/24/testing-bit-rot/ ) >> +zfs has working raid56 >> -modules out of kernel for license incompatibilities (a big minus) >> >> BTRFS can detect bitrot but... are we sure it can fix it? (can't seem >> to find any conclusive doc about it right now) > > Yes. Active fixups with scrub since 3.19. Passive fixups since 4.12. Presumably this is dependent on checksums? So neither detection nor fixup happen for NOCOW files? Even a scrub won't notice because scrub doesn't attempt to compare both copies unless the first copy has a bad checksum -- is that correct? > >> I'm one of those that is waiting for the write hole bug to be fixed in >> order to use raid5 on my home setup. It's a shame it's taking so long. > > For what it's worth, the write hole is considered to be rare. > https://lwn.net/Articles/665299/ > > Further, the write hole means a) parity is corrupt or stale compared > to data stripe elements which is caused by a crash or powerloss during > writes, and b) subsequently there is a missing device or bad sector in > the same stripe as the corrupt/stale parity stripe element. The effect > of b) is that reconstruction from parity is necessary, and the effect > of a) is that it's reconstructed incorrectly, thus corruption. But > Btrfs detects this corruption, whether it's metadata or data. The > corruption isn't propagated in any case. But it makes the filesystem > fragile if this happens with metadata. Any parity stripe element > staleness likely results in significantly bad reconstruction in this > case, and just can't be worked around, even btrfs check probably can't > fix it. If the write hole problem happens with data block group, then > EIO. But the good news is that this isn't going to result in silent > data or file system metadata corruption. For sure you'll know about > it. If I understand correctly, metadata always has checksums so that is true for filesystem structure. But for no-checksum files (such as nocow files) the corruption will be silent, won't it? Graham