Hi On 1/19/19 2:21 PM, Basil Mohamed Gohar wrote: > I have two drives of the 4-array RAID6 visible, but no files are > accessible because it's a RAID6, I need at least 3 of the 4 drives > working, and my problem is two are experiencing this problem. Hmm, that would be surprising, as RAID6 should offer a two disk redundancy, i.e. any two disks may fail and you should still be able to access your data - albeit without any extra safety net. > This is challenging because it is in a tower array and all the drives > connect straight to motherboard-like backplane. I took one out and was > working with it directly via a USB SATA adapter, but that did not change > the errors I was seeing. OK, I just wanted to make sure that the error "stayed" with the drives. > Yes, they are. SMART reports no fatal errors on the drives in questions! OK, at least that. > What may help me is if there are any tools for md devices that let me > peek into the on-disk structure. Since the ext4 file system is spread > across the 3 data drives in the array, I cannot use, for example, e2fsck > on just one of them, and since I cannot properly assemble the drive, I > am somewhat stuck. Are there any tools for examining an array of drives > even if it is not recognized as such? I don't know, for example, if some > sectors went bad, how to tell mdadm to look in alternate locations > (i.e., akin to ext4's alternative superblock locations). As indicated above with RAID6 you should "only" have two data disks in a four disk RAID6, as RAID6 does not write data copies but "generated" parity stripes to the two extra disks, it can compute back what should have been on data stripes on failed disks. But reverse engineering this is probably not really easy to perform "manually". Thus, at first, we should really establish what the underlying layout was, i.e. can you send us the output of /proc/mdstat? Cheers Carsten -- Dr. Carsten Aulbert, Max Planck Institute for Gravitational Physics, Callinstraße 38, 30167 Hannover, Germany Phone: +49 511 762 17185
