[linux-lvm] LVM label lost / system does not boot

[Andreas Schild <andreas@soulboarder.net>]

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Hi
The first part might sound like I am in the wrong group, but bear with me...
(I probably am, but I googled up and down RAID and LVM lists and I am still
 stuck):
I have a software RAID 5 with 4 disks and LVM on top. I had one volume group
with two logical volumes (for root and data).
I wanted to upgrade capacity and started by failing a drive, replacing it
with a bigger one and let the RAID resync. Worked fine for the first disk.
The second disk apparently worked (resynced, all looked good), but after a
reboot the system hung.
After some back and forth with superblocks (on the devices, never on the
array) I was able to re-assemble the array clean.
The system still does not reboot though: "Volume group "cmain" not found".

I booted a live cd, assembled the array and did a pvck on the array
(/dev/md0):
"Could not find LVM label on /dev/md0"
pvdisplay /dev/md0 results in:
  No physical volume label read from /dev/md0
  Failed to read physical volume "/dev/md0"

I do not have a backup of my /etc/ and therefore no details regarding the
configuration of the LVM setup (yes, I know...)
All I have of the broken system is the /boot partition with its content

Several questions arise:
- Is it possible to "reconstitute" the LVM with what I have?
- Is the RAID array really ok, or is it possibly corrupt to begin with (and
the reason no LVM labels are around)?
- Should I try to reconstruct with pvcreate/vgcreate? (I shied away from any
*create commands to not make things worse.)
- If all is lost, what did I do wrong and what would I need to backup for a
next time?

Any ideas on how I could get the data back would greatly be appreciated. I
am in way over my head, so if somebody knowledgeable tells me: "you lost,
move on" would be bad, but at least would save me some time...

Thanks,
Andreas

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