CentOS - Nov 2008 - Reassemble software RAID

I have a machine on CentOS 5 with two disks in RAID1 using Linux software 
RAID. /dev/md0 is a small boot partition, /dev/md1 spans the rest of the 
disk(s). /dev/md1 is managed by LVM and holds the system partition and 
several other partitions. I had to take out disk sda from the RAID and low 
level format it with the tool provided by Samsung. Now I put it back and 
want to reassemble the array.
Machine boots fine from sdb and I was able to re-add /dev/sda1 to /dev/md0 
easily. However, I can't do the same with /dev/sda2 to /dev/md1 because 
there's no /dev/md1 anymore and mdadm tells me it doesn't have the RAID 
superblock.
If possible I want to avoid deleting the already existing setup and data.
I'm wondering what I shall do next. Can I simply create /dev/md1 as if it 
was a new array? Is the problem created by the fact that the running 
system was booted from a logical volume on /dev/sda2? Should I better boot 
the system from a live CD and then reassemble/create anew /dev/md1?

In general, would it be better to *not* put the main system on a logical 
volume on a RAID partition? (The other logical volumes carry virtual 
machines.) Thus:
/dev/md0 with /boot
/dev/md1 with / (no LVM on it)
/dev/md2 managed by LVM


Kai

-- 
Kai Sch?tzl, Berlin, Germany
Get your web at Conactive Internet Services: http://www.conactive.com

It's obvious but I should have mentioned it nevertheless that I copied the 
partition table from sdb back to sda, of course.
I wonder how the superblock for /dev/md1 could be missing?

Kai

-- 
Kai Sch?tzl, Berlin, Germany
Get your web at Conactive Internet Services: http://www.conactive.com

I'm still having a problem with this software-RAID setup. I restructured 
the disk layout, so that I have the exact same partition layout on both 
disks. But now I cannot boot from the new md device.
The important devices are:

/dev/md0 with /boot (made from sda1 and sdb1)
/dev/md2 with / (made only from sda1)
/dev/sdb2 with / (to be added to md2 once booting from md2 works)

The problem is that if I boot the kernel into /dev/md2 it errors out with 
"cannot mount /dev/root" when switching root. I can boot fine by using
the
/ filesystem on /dev/sdb2. initrd contains raid1.ko, so the mdraid driver 
is not missing driver.

Any hints what could be wrong?


Kai

-- 
Kai Sch?tzl, Berlin, Germany
Get your web at Conactive Internet Services: http://www.conactive.com