[linux-lvm] Internal Knowlegde required. Please help me recover this sucker.

oliver oliver at are-b.org
Thu Mar 3 22:49:09 UTC 2005

WAS: Re: [linux-lvm] Partition table gone? Any way to restore?

After further examination the 'hostname<someserialnumber>' I've mention 
that worried me at first appears to be a unix timestamp. I don't think 
that it matters much. I've noticed that on my other LVM setup where I 
most likley did a pvcreate /dev/hdg /dev/hdi (to create them 
simultaniously) since the timestamps are 100% identical.

So I think I can be pretty safe by using the timestamp on my hde that I 
have on my hdg. I did create them the same day, I'm sure of that. (I am 
using LVM to span across multipledisks after all) The 3rd disk was added 
later, hence the different timestamp. So all I really need to know if 
some kind of crc/hash is computed after 0x0001000 so that I can try this.


oliver wrote:

> Small update. Using a hex editor i was able to more closley examine my 
> disk(s). Since I have 2 LVM setups I was somewhat able to compare what 
> I have to what I should have. I do still need some insite however.
> From what I can tell, pvcreate only modifies the first few k's of the 
> disk, followed by information stored from the vgcreate.
> Then the lvcreate adds some more information after this. (vg00/lv 
> basicly).
> I hope i got it right so far : )
> Now, the vgcreate information is identical on the two surviving parts 
> except for a 'hostname<someserialnumber>' part around address 0x00012c.
> Then at 0x0001000 we see the UUID of the VG passing by followed by 
> some data (which is identical on all three vgs.
> I don't think i have to look beyond this point in space/address.
> Assuming that none of the tools (besides pvcreate) write anything 
> before 0x0001000, I should be able to pvcreate /dev/hde edit the UUID 
> of that specific PV (to the same stored in the other two PV's so the 
> three match up again) and have a fully workable LVM set again.
> The only thing that might worry me is crc information stored somewhere 
> after 0x0001000 (However it appears to me that there isn't a whole 
> bunch of data stored before 0x0001000 and the data created by pvcreate 
> is identical on all 3 drives with the exception of the UUID so the crc 
> value should match again) and the string 'vg00'at 0x0000ac i see on 
> all disks (the name of the vg.
> What my question is (without spending days reading the sourcecode : ) 
> is am I correct? Assuming that all the 'important' metadata and such 
> is stored 'after' 0x0001000 hex, is there a good chance of it working?
> I'm thinking of running a pvcreate, change the UUID of the PV to what 
> I expect it to be, add 'vg00' at 0x000ac and be happy?
> I could simply try, I know, but some feedback first would be 
> appreciated ; ) I really don't want to loose my data.
> Thanks a lot.
> Oliver
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