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Linux md raid superblock name

From: Sean Loaring
Subject: Linux md raid superblock name
Date: Mon, 11 Nov 2013 15:13:06 -0700

Hi there,

I'm in the process of transferring a Debian install from a single drive to a md raid-1 /boot + md raid-1 LVM setup. 

When I created the /boot array I manually assigned the device md179.  Upon rebooting the array was automatically assembled under md126.

When I run grub-install /dev/sda I get the message "Auto-detection of a filesystem of /dev/md126 failed".

I downloaded grub2-1.99, compiled with -ggdb and eventually found that grub-probe extracts the name field from the mdadm 1.0 raid superblock and uses that name to determine the name of the md device.  The name of the device in my case is, according to mdadm -Q --detail, "RIPLinux:179". 

So, when running "grub-probe --device /dev/md126" the program correctly identifies the devices /dev/sd{a,b}1, retrieves the superblock from /dev/sda1, extracts the name "RIPLinux:179" and uses the number 179 to "incorrectly" determine that the md device is /dev/md179.  It then attempts to access /dev/md179 and fails.

The mdadm man page seems to suggest that the superblock name is arbitrary as long as it is unique to a set of partitions that participate in an md raid.  I didn't see anything that suggests that the superblock name necessarily has anything to do with the md device that the raid is assembled under.

I'm pretty sure that I can get grub2 installed simply by changing the name field in the superblock via mdadm --update.  However, it seems to me that either I am ignorant of the actual meaning of the superblock name field or grub2 is making incorrect assumptions.

Any illumination would be appreciated.


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