lvm over raid confusion

[Daniel Buggie]

Jack Tanner wrote:

> I think I may have just painted myself into a corner. Could someone 
> who knows LVM and software RAID look this over?
>
> I installed FC5 with the default partitioning settings (/boot + LVM 
> partition containing LVs for / and /home) on /dev/hda. Then I added 
> two more drives to the box, and did
>
> # mdadm --create /dev/md0 -l raid1 -N 2 /dev/hdb /dev/hdc
>
> Then I used system-config-lvm to join /dev/md0 to the existing volume 
> group, and moved the extent containing /home to reside on /dev/md0. 
> The idea was that I want my data on redundant disks, but I don't care 
> about the OS itself, because that can be trivially reinstalled. 
> Everything worked just fine.
>
> Then I rebooted, and the kernel halted on boot ...
> Redhat nash starting
> Couldn't find device with uuid ...
> Couldn't find all physical volumes for volume group VolGroup00
>
> I can boot off a rescue CD, and do
>
> # mdadm -A /dev/md0 /dev/hdb /dev/hdc
> mdadm: /dev/md0 has been started with 2 drives
>
> After that, I can see via lvm that all the VGs and LVs are there.
>
> So it seems like /dev/md0 isn't getting assembled on boot up. I tried 
> following the mdadm man page and had grub pass the kernel the 
> parameter md=0,/dev/hdb,/dev/hdc , but that did absolutely nothing.
>
> So, basically, I have two questions. First, how do I go about 
> diagnosing this and getting the box to boot up right?
>
> Second, am I being an idiot in the first place by making my VG contain 
> a single drive with the OS and a software RAID1 array with /home? That 
> is, if the non-redundant OS drive fails, will I still be able to plop 
> the OS on a new drive, and join the RAID1 array (or even just one of 
> the two drives in the array) to the new drive's VG?
>
>
Two things come to mind that could cause that:

1) First, for Software RAID I've always created a partition and set it 
to type fd rather than use the device itself.

2) If you create a RAID disk after installation, you need to create a 
initrd image (mkinitrd) that supports software RAID.

Hope this helps,
Daniel