CentOS - Mar 2013 - "Can't find root device" with lvm root after moving drive on CentOS 6.3

I have an 8-core SuperMicro Xeon server with CentOS 6.3. The OS is 
installed on a 120 GB SSD connected by SATA, the machine also contains 
an Areca SAS controller with 24 drives connected. The motherboard is a 
SuperMicro X9DA7.

When I installed the OS, I used the default options, which creates an 
LVM volume group to contain / and /home, and keeps /boot and /boot/efi 
outside the volume group.

The machine is a couple of months old, and has been stable. While 
installing some new hardware, I decided to also clean up the cabling in 
the box, since it was a bit messy. In doing this, I probably moved the 
boot SSD disk to another port on the motherboard (it has a bunch, 2 SATA 
6GBps and 6 SATA 3GBps).

When I booted the box after this, I got a kernel panic, the typical 
"Can't find root device".

I read some docs, and first tried to boot from a rescue disc and 
reinstal GRUB, but that didn't change anything. Further Googling got me 
the rdshell kernel parameter, and that dropped me to a shell when it 
failed to find the root device.

Reading https://fedoraproject.org/wiki/How_to_debug_Dracut_problems , I 
did the following:

# lvm vgscan
# lvm vgchange -ay

And then

# ln -s /dev/mapper/<volumegroup>-<root_volume> /dev/root
# exit

After this, the box boots up normally, and everything works as it 
should. However, when I reboot, it again fails to find the root device.

So, after all this, my question is, how do I make Dracut (I'm assuming) 
understand that this LVM volume is my root device and pick it up 
automatically?

And, is there a way to avoid this problem in the future, if I move 
drives around? Surely it can't be normal for this to happen just because 
I connect a drive to another port?

-- 
Joakim Ziegler  -  Supervisor de postproducci?n  -  Terminal
joakim at terminalmx.com   -   044 55 2971 8514   -   5264 0864

> When I booted the box after this, I got a kernel panic, the typical
> "Can't find root device".
> Reading https://fedoraproject.org/wiki/How_to_debug_Dracut_problems , I
> did the following:
>
> # lvm vgscan
> # lvm vgchange -ay
>
> And then
>
> # ln -s /dev/mapper/<volumegroup>-<root_volume> /dev/root
> # exit
>
> After this, the box boots up normally, and everything works as it
> should. However, when I reboot, it again fails to find the root device.
>
> So, after all this, my question is, how do I make Dracut (I'm assuming)
> understand that this LVM volume is my root device and pick it up
> automatically?
What does your kernel line in grub look like?

Barry

On 03/22/2013 08:27 PM, Joakim Ziegler wrote:
> So, after all this, my question is, how do I make Dracut (I'm assuming)
> understand that this LVM volume is my root device and pick it up
> automatically?
I've looked through Dracut trying to spot circumstances that might cause 
the problem that you've described, but came up with nothing.  udev 
should be scanning block devices as they become available, and setting 
up any logical volumes on all of the available block devices.

It may be useful to capture some information in the debugging shell, 
before running vgscan.

As suggested in the fedora debugging document, capture the output of the 
following commands to get a better idea of what the kernel knows about 
block devices before you manually start the volumes, and maybe that'll 
lead us to some conclusion about why the devices aren't found.

lvm pvdisplay
lvm vgdisplay
lvm lvdisplay
blkid
dmesg