Kernel disc enumeration

Letharion · Veteran Joined: 13 Jun 2005 Posts: 1344 Location: Sweden

Sometimes, after performing an install and rebooting, the newly installed kernel enumerate discs differently from the livecd one. Why is that? Can I foresee the change? Normally it's not much of a problem, but recently I performed a remote install and because sdb became sda. My carefully written and double checked grub and fstab became invalid.
I had to walk a collegue through fixing it over the phone.

Nacon · n00b Joined: 01 Jan 2010 Posts: 28 Location: germany

You can replace your /dev/sda1 with the UUID of the disk to avoid the usage of /dev/sda or /dev/sdb.

Just get the UUID by running the "blkid" command:

Letharion · Veteran Joined: 13 Jun 2005 Posts: 1344 Location: Sweden

Thanks, that's useful to know

Is this the "recommended" way of solving this issue?
It doesn't shed any light on why thet problem exists in the first place.

d2_racing · Posted: Sun Mar 07, 2010 8:18 pm Post subject:

Never used that actually.

NeddySeagoon · Posted: Sun Mar 07, 2010 8:21 pm Post subject:

Nacon,

That doesn't work for me but my root is /dev/md3 which is a raid5 volume.
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NeddySeagoon

Computer users fall into two groups:-
those that do backups
those that have never had a hard drive fail.

Sadako · Posted: Sun Mar 07, 2010 8:45 pm Post subject:

Isn't the UUID a filesystem attribute rather than a disk identifier?

I mean, run mkfs on the same partition and you'll have a different UUID.

I use the /dev/disk/by-id/* symlinks within fstab, which offer more or less the same functionality as UUIDs, only you can't use them to specify the root block device to the kernel.
It's enough to save against "/dev/sda" and "/dev/sdb" switching, for example.

NeddySeagoon · Posted: Sun Mar 07, 2010 8:55 pm Post subject:

Hopeless,

The array is autoassembled and that still happens before the kernel tries to mount root.

Sadako · Posted: Sun Mar 07, 2010 9:08 pm Post subject:

NeddySeagoon · Posted: Sun Mar 07, 2010 9:20 pm Post subject:

Hopeless,

I'll add in things on at a time, so I can test at normal boot time.
I've made some notes in grub.conf.

Can I use UUID to manually assemble the raid5?
If I have to use device names before I can use UUID, that defeats the purpose.
_________________
Regards,

NeddySeagoon

Computer users fall into two groups:-
those that do backups
those that have never had a hard drive fail.

Sadako · Posted: Sun Mar 07, 2010 9:31 pm Post subject:

NeddySeagoon · Posted: Mon Mar 08, 2010 8:18 pm Post subject:

Hopeless,

It looks like you are right about UUIDs being filesystem attributes.

Also, rootdelay=10 with UUID and root on raid5 doesn't help. It only takes 10 seconds longer to work out that it can't mount root.
The error message listed all the partitions with their drivers, except for the raid sets which had their drivers listed as (??).

I'll try the manual assembly next.
_________________
Regards,

NeddySeagoon

Computer users fall into two groups:-
those that do backups
those that have never had a hard drive fail.

krinn · Watchman Joined: 02 May 2003 Posts: 7470

read somewhere you must set the UUID in mdadm.conf too, and use blkid /dev/md? to get UUID of the assembled array to put in mdadm.conf.

NeddySeagoon · Posted: Tue Mar 09, 2010 8:44 pm Post subject:

krinn,

Thats PITA. It means I have to have an initrd as I have root on raid and mdadm.conf is on the root fs.
I kept root outside of LVM to avoid having to deal with an initrd.

I'll play with UUIDs on my netbook
_________________
Regards,

NeddySeagoon

Computer users fall into two groups:-
those that do backups
those that have never had a hard drive fail.