August 4, 20205 yr 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
August 4, 20205 yr Community Expert 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.
August 4, 20205 yr Author 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
August 4, 20205 yr 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.
August 5, 20205 yr Community Expert 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
November 2, 20205 yr Author 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
November 3, 20205 yr Community Expert 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.
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