[linux-lvm]VG Recovery off multiple PV's

Tom Hollins tom at tomhollins.com
Sun Feb 8 22:09:09 UTC 2004

Whoops! skipped the step of figuring out where the partition starts (SORRY!). I was putting the kids to bed.
Load the dd'd partition file (hdg1.e2fs) into hexedit (hexedit hdg1.e2fs) 
Hold <CTRL> key and press <S>
Enter 53EF  press <return>
You will need to eyeball this. Whereever you end up, back up 56 bytes. Most likely this 
will be a 0x00 and previous area for the previous 0x400 (1024 bytes) should be zeroes (this would have been your
MBR area). Mark the start of this (Address of 53ef) minus (56 bytes) minus (0x440 bytes) and this will be the start of your ext2 partition. That will be used to help calculate the skip value for dd.
So the address is in hex, multiply by 16 to turn into decimal divide by 512 (your block size) and this
is the number of input blocks to skip before dd starts copying.

Additional data I found about the LVM partition on the disk itself:
1) 0x00 - 0x1FFFF - LVM data
2) 0x20000 - 0x2E50F - Physical extents region as reported by pvdata. Notice all of the 0200s, remember this is in byte reversed order, so this is the vg2 physical extents data, then vg3 etc...
3) 0x2E510-0x401FF - all zeroes (probably the end of the physical extents table
4) 0x40200-0x41fff - un-initialized part of the disk.
5) 0x525000 -        My ext3 partition. Yours might be the same since I had a 60G disk also.

I am sure the LVM programmers will get a recovery utility going when time permits or someone else will write one, but for now this is all I gleaned from 16 hrs searching the net, reading the entire lvm-dev mailing list and all.


On Sun, 08 Feb 2004 20:35:19 -0500
tom <tom at tomhollins.com> wrote:

> I saw your message in January but I have been working on the "mysteries 
> of LVM on-disk layout". Here is what I "think" I've found.
> 1) Even if your lvm is screwed up, your ext2/3 data is probably still 
> there since that volume wasn't the one with physical problems.
> 2) If you want to "backup" for safety's sake your ext2 data then you 
> need to do a pvdata for each of your partitions. The developers have a 
> better handle on this, maybe one day they will post to your problem.
> The Ext2 filesystem is placed on a hard disk starting at 1024 bytes in 
> from the beginning of the partition. This is probably for MBR areas 
> (which are usually 512 bytes I believe).
> There is a 56 byte data area (mine always start with 0x00)
> There is the magic number for the filesystem type 53EF (this is byte 
> reversed order of 0xEF53. Magic numbers found in /usr/share/misc/magic)
> The there is your ext2 partition metadata and data (I don't care of the 
> layout and neither do you, only the size).
> Question is where is this on the partition? And the answer is, this is 
> roughly dynamically assigned space based on the size of your physical 
> volume (I am guessing, you have to look at the lvm code). I used dd to 
> find out using hexedit what is going on here :
> I will use your first partition /dev/hdg1 to work on all examples.
> dd if=/dev/hdg1 of=(any filename on a partition with space to hold it 
> lets call it hdg1.e2fs for example) ibs=512 obs=1M count=(supposed to be 
> ibs * this count=total bytes but I didn't have the space for it on mine 
> so I can't be sure)
> Your data is saved. Now the hard part is to either mount it in the 
> loopback mount, note: the directory name can be anything.
> mkdir /mnt/widget  
> mount -t ext3 -o ro,errors=continue,loop  (full path)/hdg1 /mnt/widget
> Now mine says the standard error of bad superblock, etc... And this is 
> as far as I got. Maybe you won't have my problems.
> Also, my theory is that if you have a comparable disk and lets say it is 
> hdf then you should be able to dd directly to that disk and it should 
> mount ok (my theory only but I am going to try it when my drive 
> arrives). the command would be :
> dd if=/dev/hdg1 of=/dev/hdf ibs=512 obs=1M count=60G
> About 45mins later roughly it should be done. There is NO status info in 
> dd saying nn% complete or anything like that, just let it run.
> After all data is accounted for and backed up then you can restart the 
> LVM process.
> I hope this helps a little, sorry I haven't the experience (yet) to tell 
> you how to finish the recovery.
> I want to say though, that as you start your LVM init process for the 
> disks that the lvm dir should be backed up at each step then reboot to 
> make sure everything is ok. Short of this and you will get yourself into 
> my situation, where there is data and preferences that have accummulated 
> over time that I never thought to backup, I copied the data to the lvm, 
> everything looked ok, booted ok, then BLAM primary boot disk dies and 
> takes all of the LVM data with it, with nearly no chance of recovery. 
> C'est la vie!
> Good luck. Happy hunting.
> -T-
> _______________________________________________
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> linux-lvm at redhat.com
> https://www.redhat.com/mailman/listinfo/linux-lvm
> read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/

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