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Re: lvm over raid confusion



On Sun, 2006-03-26 at 21:58 -0800, Daniel Buggie wrote:
> 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.

I used the whole device once and utterly confused myself when at a later
date I was trying to resolve an unrelated boot problem and got messages
about being unable to find a partition table on that device. I had a bit
of a penic for a while until I realised there never was a partition
table on it. Ever since then I've made a partition table and one big
RAID partition even if I want to use the whole disk.

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

Yes, that's probably the problem here. When the initial install was
done, there were no RAID devices and so the drivers weren't included in
the initrd.

# mkinitrd -v -f /boot/initrd-2.6.15-1.2054_FC5.img 2.6.15-1.2054_FC5

Paul.


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