Re: [linux-lvm] Recovering PV's VG metadata

[HopNet <hne@inetnow.net>]

Thanks to Heinz & Andreas for your suggestions.  They are "almost" working. :-)  I can activate the VG, but I'm getting "wrong major or minor number" from the mount of the lvol, so I'm thinking I have an offset that is not correct.

Heinz, you're assumptions (listed below) are correct.  The PVs are identical models.

One thing I have a question about, and this may lead to resolving my offset problem, is where does the following pvdata info come from?
hda showed: (after LILO, which, I can see from the hex dump, was indeed the problem)
PV Size               12.14 GB / NOT usable 3.21 MB [LVM: 16.09 MB]
hdc showed:
PV Size               12.14 GB / NOT usable 3.24 MB [LVM: 242 KB]

After I copied the header over, they are the same (as hdc).  Should they be the same?  Or do I need to fudge some more numbers in the hda header.

Heinz, from your paragraph below: What should I be looking for to identify the first sector of "?"  Is there a block diagram available with the metadata layout?

Andreas, this appears to be a v1 header, so no uuid.  This confused the heck out of me for a while, until I finally deciphered part of lvm.h.  Also, pvdata doesn't support -PU on this release.

If I can't get the offset fixed, I'll probably try creating the "lvrecover" that Andreas suggested.

One the bright side, I do have a backup I recovered!  On the dark side, my backups have been flaky, and the only good one was two months old :-(  Time to invest in a new tape drive.  Well 12G of data is better than no data at all.

Heinz J. Mauelshagen wrote:

> I understand:
> 
>  - you are not booting from hda or hdc
> 
>  - hdc still holds a valid LVM VGDA
> 
>  - likely the first sector of hda got blown away (by lilo)
> 
>  - you don't have any /etc/lvmconf/ VGDA backup files on disk/tape
>    (if not so use vgcfgrestore(8) in order to restore metadata to /dev/hda!)
> 
> 
> I'm assuming based on the data below:
> 
>  - your 2 physical volumes are equal in size
> 
>  - you had just 1 logical volume spread over both physical volumes using
>    all of the VG's capacity
> 
>  - all of your VGDA with the exception of the physical volume structure,
>    which was sitting at the very beginning of hda is still there and
>    likely valid
> 
> 
> *If* the above assumptions are correct, your option is to copy the first
> sector of /dev/hdc over to /dev/hda with
> "dd if=/dev/hdc bs=512 count=1 of=/dev/hda" *and* change it with a hex editor.
> 
> In order to find the correct offsets into the first sector on /dev/hda to
> change, look at lvm.h (of LVM 0.8final!) and the definition of pv_disk_t in
> that header file.
> 
> At least you need to change the physical number (pv_number); set it to 1.
> 
> In case the above assumptions are not correct for eg. the sizes of the PVs
> differ, you need to change pv_size, pe_total and pe_allocated as well.
> 
> Please get back to me if this is the case.
> 
> 
> BTW: we are working on the enhancement of our LVM checker in order
>      to support such repairs. Not very helpfull for you nor, I know :(
> 
> Don't forget to check your /etc/lilo.conf to make sure, that lilo doesn't
> tamper with the first sector on /dev/hda again!