I'm replying to everyone in the same email. On Wed, 4 Jun 2025 08:34:35 -0400 "John Stoffel" <john@xxxxxxxxxxx> wrote: > >>>>> "David" == David Niklas <simd@xxxxxxxxxxx> writes: > > > My PC suffered a rather nasty case of HW failure recently where the > > MB would break the CPU and RAM. I ended up with different data on > > different members of my RAID6 array. > > Ouch, this is not good. But you have RAID6, so it should be ok... > > > I wanted to scan through the drives and take some checksums of > > various files in an attempt to ascertain which drives took the most > > data corruption damage, to try and find the date that the damage > > started occurring (as it was unclear when exactly this began), and > > to try and rescue some of the data off of the good pairs. > > What are you comparing the checksums too? Just because you assemble > drives 1 and 2 and read the filesystem, then assemble drives 3 and 4 > into another array, how do you know which checksum is correct if they > differ? Once I find some files whose checksums differ, I can perform some automated data tests to find which file is the intact one. > > So I setup the array into read-only mode and started the array with > > only two of the drives. Drives 0 and 1. Then I proceeded to try and > > start a second pair, drives 2 and 3, so that I could scan them > > simultaneously. With the intent of then switching it over to 0 and > > 2 and 1 and 3, then 0 and 3 and 1 and 2. > > I'm not sure this is really going to work how you think.... <snip> I just think that I'll be able to read from all 4 drives but doing it in 2 arrays of 2 drives. Basically, I'll get a 2x speed increase over doing it as 2 drives at a time. On Wed, 4 Jun 2025 07:22:15 +0300 Jani Partanen <jiipee@xxxxxxxxxxx> wrote: > On 03/06/2025 23.04, David Niklas wrote: > > I think you misunderstood my original question, how do I assemble the > > RAID6 pairs (RO mode) into two different arrays such that I can read > > from them simultaneously? > > I dont think there is any other way to do what you want to do than use > overlayfs. You may find some ideas from here: > > https://archive.kernel.org/oldwiki/raid.wiki.kernel.org/index.php/Irreversible_mdadm_failure_recovery.html Thanks for the idea. I'm not following why we setup the overlay but then use mapper devices (which came from where?), with the mdadm commands. On Tue, 3 Jun 2025 21:27:35 +0100 anthony <antmbox@xxxxxxxxxxxxxxx> wrote: > On 03/06/2025 21:04, David Niklas wrote: > > Searching online turned up raid6check. > > https://unix.stackexchange.com/questions/137384/raid6-scrubbing- > > mismatch-repair > > > > But the people there also pointed out that Linux's raid repair > > operation only recalculates the parity. I would have thought that it > > did a best of 3 option. I mean, that's a big part of why we have > > RAID6 instead of RAID5, right? > > From what I remember of raid6check, it actually does a proper raid 6 > calculation to recover the damaged data. > <snip> > I've done it slightly differently, I've got raid-5 sat on top of > dm-integrity, so if a disk gets corrupted dm-integrity will simply > return a read failure, and the raid doesn't have to work out what's > been corrupted. I've got a different problem at the moment - my array > has assembled itself as three spares, so I've got to fix that ... :-( > > Cheers, > Wol Good to know. Thanks Wol. I hope you're able to get your drives up and running again. On Wed, 4 Jun 2025 10:59:21 +0200 Reindl Harald <h.reindl@xxxxxxxxxxxxx> wrote: <snip> > > as I don't have enough room otherwise > > seriously? > > an external 10 TB disk costs around 200 EUR > an external 20 TB disk costs around 400 EUR <snip> Every time I upgraded the size of my array, I'd take the old disks and use them as backup disks. Over time, it became a matter of not having enough SATA ports, not a matter of costing too much. I was trying to reuse disks instead of the disks being tossed out or collecting dust. I've learned better now. Thanks, David
