[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.
--
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