[lug] help needed with LVM situation

Nate Duehr nate at natetech.com
Wed Jun 13 03:25:50 MDT 2007

A friend of mine recently passed away, and another friend has been  
painstakingly working on recovering some important data from one of  
his machines.

A short re-cap:

He had two systems online at his home.  We'll call them system A and  
system B.

The folks trying to recover data from them had numerous problems  
facing them:
1. No one had any of the passwords.
2. One machine NFS mounted the other.
3. Calling them, machine A and machine B... Machine B had a  
motherboard failure within a day or two of our mutual friend's passing.
4. Both machines were (thankfully) set up with non-LVM root filesystems.

So they've been working on it for a while now, they're not Linux  
admins, but they work on Unix at work as Engineers for a living.   
They got Machine A online and figured out how to get into it by  
mucking around with the root password since they had physical  
access.  No problem.  Just a learning curve.

Then they found that Machine A had a bunch of hard-coded NFS mounts  
to Machine B which was off-line, and contained the CVS repository and  
other things they were struggling to recover.

They placed the hard disk from Machine B into Machine A and attempted  
to figure out how to mount it, thinking they could just grab a quick  
backup... and they found... LVM.  Actually, as I talked to one of my  
friends about this on the phone tonight, they found both machines  
were set up with LVM.  None of them had ever used LVM before, or had  
to deal with it.  Again, a learning curve, but not insurmountable (no  
pun intended).

We also attempted this evening, simply mounting up /etc off Machine B  
and wiping out the root password, and then booting from that disk.   
What we found was that the LVM tools are complaining that they're  
"not the right version" for the LVM currently on the disk.  I  
*think*, but haven't had a chance to prove (hard to do this on the  
phone) is that Machine A's LVM version is higher than Machine B's,  
and it "did something" to the LVM VG it found suddenly on /dev/hdb3  
when the disk was added.

Current status:

We figured out that both machines had single hard disks, and  
apparently both had some version of Debian LVM on them.  They may not  
have been on the same release of Debian, nor using the same version  
of LVM, but initial digging didn't turn up much.  We can get the LVM  
version (lvm10 package is installed on Machine A) from Machine A.

We also figured out, and here's the annoying part... both VG's are  
named the same.  If you use the LVM tools to look at things (trying  
not to trash the only copy of this data), you see there's a PV in / 
dev/hdb3 (when Machine B's disk is physically in Machine A as /dev/ 
hdb), and the VG is named the same as the currently active VG on  
Machine A... vg00.

(There's a case to be made here for never accepting default names for  
VG's, I think... if one machine had machineA_vg00 as it's VG name,  
and the other machineB_vg00, this wouldn't be a problem.)

To add insult to injury, it would appear that both machines are using  
ReiserFS for their filesystems in their VG's.

With the recent Linux RAID discussions, I have this gut feeling that  
tells me there are some real LVM gurus lurking on this list.  What  
say you?  How would you get this silly thing mounted?

One thought we've had, but it was getting late and messing with disks  
and LVM late at night when everyone's tired and it's the only known  
copy of the data, is a bad idea...  was to rename the VG that's  
currently active on the working Machine A.   Then there would be no  
namespace clash between the two.

The other thought that seemed saner, and slightly safer, would be to  
load a completely clean copy of Debian Sarge up on another machine,  
install the disk from Machine B into it, and then just activate the  
VG there.  Personally I like that idea better, but it is more  
difficult (since no one has any "extra" machines lying around at the  
moment -- or if we do, we're all separated by at least 50 miles at  
our homes or more).

So I'm appealing to the LVM gurus here... any thoughts?

Of course, for all of us, this is a good example of why to keep known- 
good onsite and offsite backups that are easily readable by  
anything... CD/DVD comes to mind here.

What it makes more clear even than that, is if there's someone elses  
important data on any of your machines, a good document kept with  
your will or whatever... somewhere someone would find it... that  
explains your backup scheme, perhaps has your root passwords or  
instructions about how to break into your machines, etc... is  
probably not a bad idea for anyone who has to struggle through losing  
a friend, and at the same time has to try to retrieve any of your  
"digital life".

A Digital Systems Will, if you like.  Who gets the drives, how do  
they recover the data and who's responsible to distribute the data if  
multiple people need copies for themselves, if your machines ever go  
silent... because you did.  The named executor of your "Digital Estate"?

Nate Duehr
nate at natetech.com

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