[CentOS] OT: Fortunate clueless dd chum - lvm recovery

Sat Aug 15 02:15:12 UTC 2009
Ross Walker <rswwalker at gmail.com>

On Aug 14, 2009, at 9:22 PM, Robert Nichols  
<rnicholsNOSPAM at comcast.net> wrote:

> Ross Walker wrote:
>> On Aug 14, 2009, at 12:48 PM, Robert Nichols
>> <rnicholsNOSPAM at comcast.net> wrote:
>>
>>> Ross Walker wrote:
>>>> On Fri, Aug 14, 2009 at 5:00 AM, Chan Chung Hang
>>>> Christopher<christopher.chan at bradbury.edu.hk> wrote:
>>>>
>>>> Question now is, was the first sector of partition 1 damaged (was  
>>>> it
>>>> 63 or 64 sectors dd'd)?
>>>>
>>>> If so it will require a more tricky procedure to fix.
>>> No, the ext2 file system does not use the first 1K block of the  
>>> partition.
>>> That space is left free for a boot loader.  The first super block  
>>> of the
>>> file system is the 2nd 1K block of the partition, and even if that  
>>> gets
>>> overwritten it is easily recovered from one of the backup super  
>>> blocks.
>>
>> I did not know that, good information.
>>
>> So stage2 is written in first 1k of the partition?
>
> No, nowhere near enough room.  Just stage 1 goes into the first  
> sector.
> The second sector is unused.  Stage 1 then loads the fs-specific  
> stage 1.5
> from absolute sector numbers on the disk.  Stage 1.5 then loads  
> stage 2
> from the file system.  (Yes, if the *_stage1_5 files get moved on the
> disk, you have to re-run the GRUB installer.)
>
> The only time more than the 446-byte stage 1 gets installed is when
> installing GRUB in the MBR or in the boot sector of a logical drive in
> the extended partition.  There, there can be a full track available,
> which is enough room for the appropriate stage 1.5, but not for the
> 100+ KB of stage 2.
>
>> Ah but I believe the OP has the partition setup as a LVM PV. Question
>> is does LVM leave the first 1k free for a stage2 boot loader?
>>
>> And if the LVM metadata is corrupt is there a second copy at the end
>> of the PV?
>
> No idea.  I don't use LVM.  Having a known recovery path if things get
> munged is more important to me than the features of LVM.

Since you don't know if LVM has a recovery path how can you imply it  
doesn't?

-Ross