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

Robert Nichols rnicholsNOSPAM at comcast.net
Sat Aug 15 01:22:14 UTC 2009

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.

Bob Nichols     "NOSPAM" is really part of my email address.
                 Do NOT delete it.

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