pengrus Posted August 4, 2020 Share Posted August 4, 2020 So I searched the wiki and the forum but can't seem to find a straight answer to this. How do I locate an error in parity found during a parity check? I have dual parity, and my last check unexpectedly showed 7 errors. I have what seem to be sectors identified in the log, but isn't that on the parity drives? Thanks! -P tower-diagnostics-20200804-1548.zip Quote Link to comment
itimpi Posted August 4, 2020 Share Posted August 4, 2020 The simple answer is that you cannot All Unraid knows is that the parity calculation for a particular sector does not agree with what is stored on the parity drive. Unraid does not know which drive caused this to happen. Quote Link to comment
pengrus Posted August 4, 2020 Author Share Posted August 4, 2020 Thanks @itimpi. Perhaps I was mistaken, I thought there was some way with P+Q parity to be able to find out where the corruption (or not) was. Is this not the case? -P Quote Link to comment
testdasi Posted August 4, 2020 Share Posted August 4, 2020 1 hour ago, pengrus said: Thanks @itimpi. Perhaps I was mistaken, I thought there was some way with P+Q parity to be able to find out where the corruption (or not) was. Is this not the case? -P No. That would be mathematically impossible with just P+Q parity alone. Parity correction only applies to a (or two) self-identifying failure(s) i.e. you have to know in advance which block (in Unraid case, disk) fails. So a parity error can only tells you there is a failure and you have to go identify it yourself. You might be confusing Unraid parity with RAID + checksum filesystems (e.g. ZFS / BTRFS). So for ZFS / BTRFS, a scrub not only verifies parity but also the checksum of the blocks. So if there's a parity error, it would rely on the checksum to identify the wrong block (actually I think it works the other way round i.e. it identify checksum error on a block and rebuild parity that depends on that block). A way to accomplish what ZFS / BTRFS RAID do is to have array disks use BTRFS file systems (instead of XFS). That way when you have parity error, you can just run a scrub and find out which disk has the error and if it's serious you can then rebuild the disk. Quote Link to comment
JorgeB Posted August 5, 2020 Share Posted August 5, 2020 8 hours ago, pengrus said: I thought there was some way with P+Q parity to be able to find out where the corruption (or not) was. Is this not the case? There could be a way but it's not implemented since if there is corruption on multiple devices you would end up corrupting more disks, more info here: https://forums.unraid.net/topic/46170-unraid-server-release-620-beta20-available/page/10/?tab=comments#comment-456693 Quote Link to comment
pengrus Posted November 2, 2020 Author Share Posted November 2, 2020 Thank you all for your responses, I really appreciate it. Not trying to resurrect a dead thread here, but what would cause 7 parity errors to be found on every subsequent parity check? Ever since this happened, every monthly check has returned 7 errors. I am set to correct parity errors during a check, btw, and have been up for 65 days, the reboot was intentional and orderly. Diagnostics attached, thanks! -P tower-diagnostics-20201102-1843.zip Quote Link to comment
JorgeB Posted November 3, 2020 Share Posted November 3, 2020 8 hours ago, pengrus said: but what would cause 7 parity errors to be found on every subsequent parity check? All the checks on the diags were non correct, so it will keep finding the same errors. Quote Link to comment
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