RAID 1 error question - boot problem.

[Robin Laing]

Bill Davidsen wrote:
> Robin Laing wrote:
>> Hello,
>>
>> I am trying to help someone get a system to reboot after a system 
>> issue.    He is not the builder of the system and the person that 
>> knows the system is away for a few weeks.  Great timing.  :)
>>
>> It is Fedora 10.
>>
>> Two drives, two partitions each drive which one is a mirror.
>>
>> /dev/sda is partitioned as /boot and / which is mirror 1.
>> /dev/sdb is partitioned as swap and /
>>
>> The system wouldn't restart on the reboot and came up with an error 
>> after creating the raid arrays and then saying that it cannot find 
>> /dev/md0.  I don't have the exact error message right now.
>>
>> Using an Ubuntu disk (persons personal preference) the system was 
>> booted into a live system and using gparted the partitions were shown 
>> to be as above with as I see it, one error.
>>
>> /dev/sda1 ext3 boot
>> /dev/sda2 ext3 /   raid
>>
>> /dev/sdb1 Swap
>> /dev/sdb2 Unknown  / raid
>>
>> Running mdadm /dev/sdb2 --examine shows that the partition superblock 
>> is showing RAID 1 and that it is clean.
>>
>> As this is a critical system, it is a priority and is being used as a 
>> virtual server.
>>
>> With only the second drive installed, we tried to run fsck.ext3 on the 
>> /dev/sda2 (normally b2) with no success.  We also tried /dev/md0 as 
>> Ubuntu has created the /dev/md0 from the single drive.
>>
>> The user has not tried to boot with only the one drive in yet.  He is 
>> making a copy of the drive on a different system.
>>
>> Now, the question.  On booting from a mirror 1 array, if there is a 
>> problem with the raid system, how does the boot process read the 
>> mdadm.conf file when it is on the RAID array that needs to be created? 
>> Is there some data that is stored in the /boot or someplace else that 
>> has the necessary info to tell the system how to build the array?
>>
>> Is it part of the /boot/grub/device.map or /boot/System.map* ?
>>
>> Any suggestions to where to start?
>>
> The linux-raid group would have been a better choice, but this is a 
> simple question. The mdadm.conf file should get put in the initrd file, 
> which is in the /boot partition, which you didn't mirror for some 
> reason. I'm guessing that sda2 is a better place to start, since that's 
> recognizable as an ext3 partition. Having a partition identify as 
> "Unknown" is usually not a good thing. I would mount that partition and 
> copy the contents to a secure backup if this is critical.
> 
> "I don't have the exact error message right now" doesn't help, I suggest 
> backing up sda2, and sda1 if you can, noting the error message, and post 
> back. Without more information I am guessing that the sdb2 partition is 
> in some way hosed, do NOT run fsck on sda2 before backing up, and run 
> with the "-n" option to see what condition the f/s is in. I doubt you've 
> lost your data yet, don't do anything which would change that.
> 

Thank you for the information.

This was complicated as I was helping an admin (ubuntu user) that had 
never worked with md and was filling in for the admin that created the 
system.  I have had my share of md issues between various versions of 
Fedora over the years.  He had already started trying to fix the system 
before I even heard about the problem.  Part of the reason I didn't have 
the answers.

As it turned out, there were two problems, one on each drive.  We got 
the system up and running before there was any response to my query.

We wanted to understand more things about the md operation, especially 
on booting with md arrays and using the /boot partition.

You confirmed my suspicion that the superblock was the key.

-- 
Robin Laing