mdadm superblock problem

[drescherjm]

I had a problem with a gentoo server that has been in production for 3 to 5 years now (updated udev but forgot to remove deprecated sysfs).

I could not fix the problem in the running system so I tried to boot from a sysrescue cd and hit a big problem. The machine has 6 750GB hard drives that each have 5 partitions. 4 of these partitions are mdadm raid members. The problem is that during boot 2 of the disks were detected as whole disk members on md0 instead of /dev/sda1 and /dev/sdd1 which were partition members of the md0 raid 1 array. The problem appears that my disks have superblocks (probably from testing before depoyment) when they should not have them.

[krinn]

[drescherjm]

[krinn]

You should backup before altering your array. It's not like it's a critical thing you must do, it's a cosmetic feature you wish working with a livecd. Is it worth the 4-5TB gambling ?


If you are looking for an alternate but faster way of testing that (as secure as it could be without a backup).
I would consider picking one disk as my "test disk", of course, one that is affect by the issue.

Then i would snapshot that drive (sorry i don't have the command in mind, but i'm sure dd is a tool that can do that easy).
This way, i backup the snapshot of the drive, lowering datas to backup to the drive capacity (750G for your case), lol of course not backing that on the array itself, you need find the place elsewhere.

Then setting my array (if possible) and my gentoo to work RO on the array (trying my best to avoid the array writing infos about a failure that might coming next)
Alter the target disk with your modifications.

Now if you boot the array/gentoo and modification prevent the array from working: the array should be RO, all disks except the modified one should be ok. Then restoring (dd or other tool you use to make the snapshot) that disk should get your array back to the previous state (hmm, well, in theory)

This is tricky, this is risky, this is what i always done because i'm lazzy to backup.

As a foot note: you shouldn't asking advice from unknow user on a forum, what risk do they get? 0, i will still sleep very well if your array is dead, i might get ban as retaliation, woooooo, poor me.
But thinking about your side, you have lost 4-5G of datas, and worst you've put a production server in a stop state for some time.
So even someone here tell you : "don't worry, alter it, it's ok it will works", you should still think if anything goes wrong, and many things can goes wrong when tweaking stuff: who will face the result ? Getting fired for this kind of stuff is possible.

Krinn gains a level, wisdom+1

[drescherjm]

[drescherjm]

With the help of wikipedia I found the location of the superblock

https://raid.wiki.kernel.org/index.php/RAID_superblock_formats#The_version-0.90_Superblock_Format