[nSLUG] Device enumeration flip flop

D G Teed donald.teed at gmail.com
Wed Mar 19 15:24:42 ADT 2008


There is an Intel system I have which apparently has a bad motherboard
and there is no spare on hand, but there is a mostly similar server
spare which can take it's system disk.  We added a scsi card
to the spare so it can resume the service run from an external array.

It boots from SATA disks.

As the spare system booted from the original system disk,
we could only get this seemingly classic error:

mount: count not find filesystem '/dev/root'
setuproot: moving /dev failed: No such file or directory
setuproot: error mounting /proc: No such file or directory
setuproot: error mounting /sys: No such file or directory
switchroot: mount failed: No such file or directory
Kernel panic - not syncing: Attempted to kill init!

The disk was from a degraded Linux MD raid 1, but it
had booted fine before on the original system.  The partition
for / was changed into ext3, and no change.  It seems to be
somehow related to not getting what it needs from initrd (standard
issue from Redhat, mentioned in menu.1st), but
I don't know.

>From a rescue cdrom boot, it was clear the device
enumeration order had changed, and now sda to sdf
was the external array.  I figured e2label could address
that, but no luck - same error.

RH was installed on a fresh disk on a SATA drive /dev/sdf
It was not able to boot - no grub error.

Finally I decided to put the MBR on the first disk of the SCSI
array, to restore the service the server provides.
Using a RH rescue CD, I've put MBR on SCSI disk 1 (sda),
telling it that root is on hd5,0 (sdf1) - first SATA.

Installing grub on SCSI's first disk brought me the grub screen
but it wouldn't boot until I edited within the grub boot screen
and told it to use root(hd0,0)

When the fresh OS install booted, I want to try my original system
disk on sdg1, but now I found this was a device on SCSI.
Huh?  Somewhere along the way, what was known as /dev/sdf
from the rescue cdrom (same as installer) had turned
into /dev/sda, and /dev/sdg was now /dev/sdb.
Did I trigger this flip/flop with the use of the "device" line
in grub shell?

This presents confusion, because I don't know what disk enumeration
to use at what point in fixing our grub set up.  Labels do not
apply to grub root statements.  If I could better control the
enumeration order it would be great, but the CMOS doesn't
seem to support it, except possibly under a very complicated
feature called the "EMI shell".

So far, I was able to boot the fresh install once, but attempts to
do the same for the original OS disk have failed - with what
I believe are device enumeration issues.

Has anyone run into this kind of a challenge before, and it so,
what did you do to tame the beast?

--Donald



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