mdadm still claiming old RAID member drive on boot

octomancer · n00b Joined: 29 Dec 2006 Posts: 6

Hi,

Not sure this is the right place to ask this, apologies if not.

I had a software (mdadm) RAID5 composed of a bunch of 2TB drives in my Gentoo server. It was created a long time ago, the member disks' partition tables were MSDOS ones, a reasonable choice at the time. The partition type was set to "fd", the mdadm auto-assemble partition type. These partitions were created with fdisk. To replace this RAID5, I created a new RAID6 on 4TB drives. I had to use GPT partition tables on those due to the 4TB size. I used parted to create these partitions. Once I'd copied all the data over, I decided that I wanted to use one of the old 2TB drives as a general purpose data drive in the same server. Because I had been using parted, I used it to create a GPT partition on this old 2TB drive. Then I used mkfs.ext4 to create a filesystem. So far so good.

But now, when I boot my server, mdadm "claims" the old 2TB drive, as shown:

eccerr0r · Posted: Fri Jun 08, 2018 12:32 pm Post subject:

Yes, if md detects a superblock on any disk on the system when autodetecting, it will assume that it needs to eventually assemble it and snap it up... I believe this happens regardless of the partition type, it still seems to check them all.

You should wipe the superblock with something like

# mdadm --zero-superblock /dev/sdb

if you don't want it ever showing up again. Backup before using this command unless you want to trash the contents of the drive anyway.
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octomancer · n00b Joined: 29 Dec 2006 Posts: 6

Thanks for the quick reply.

At the moment, the RAID superblock and the ext4 filesystem are happily co-existing on that drive (/dev/sdb). Unless zeroing the superblock is going to write zeros somewhere other than the superblock, I can't see why it would trash the contents of the drive. I have a spare SATA port in the server, so I'll put another of the old 2TB drives back in it (and zero the superblock before doing anything else :-D

and change the partition type to something other than fd with fdisk before creating the GPT partition) and backup the data to it before I try zeroing the superblock on /dev/sdb. But really, if it the ext4 filesystem is happy with a valid superblock there, it should be happy without one. Ooooh, I get do science! I will report back with the results. Hopefully I don't create a monster.

eccerr0r · Posted: Fri Jun 08, 2018 12:58 pm Post subject:

Well,... you were warned. It shouldn't cause trouble, but if it does, wouldn't it be nice if you had a backup?
_________________
Intel Core i7 2700K/Radeon R7 250/24GB DDR3/256GB SSD
What am I supposed watching?

NeddySeagoon · Posted: Fri Jun 08, 2018 6:51 pm Post subject:

octomancer,

It sounds like you have a version 0.9 raid superblock on that drive.

octomancer · n00b Joined: 29 Dec 2006 Posts: 6

Hi

You're right Neddy.