[linux-lvm] recover a pv from a disk with no partition table

[linux-lvm] recover a pv from a disk with no partition table

[Brian J. Murrell]

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I need to recover a PV from a disk with no partition table.  I have a
disk which gives EIO errors trying to read from the first few blocks of,
so, no partition table.

Is there any way with LVM2, given that I have
the /etc/lvm/backup/<vgname> file that I can determine the offset into
the disk where the partition starts, so that I could do:

# losetup -o <offset> /dev/loop2 /dev/hde

to get the PV on /dev/loop2 and then be able to activate it with
vgchange and read it's LVs?

gpart tells me:

# losetup -o $((512*8)) /dev/loop2 /dev/hde
# gpart -vg /dev/loop2


dev(/dev/loop2) mss(512)

* Warning: strange partition table magic 0x0000.
...


Begin scan...
Possible partition(Linux ext2), size(19mb), offset(0mb)
   type: 131(0x83)(Linux ext2 filesystem)
   size: 19mb #s(39248) s(55-39302)
   chs:  (1023/255/0)-(1023/255/0)d (0/0/0)-(0/0/0)r
   hex:  00 FF C0 FF 83 FF C0 FF 37 00 00 00 50 99 00 00

Possible partition(Linux LVM physical volume), size(156315mb), offset(19mb)
   type: 142(0x8E)(Linux LVM physical volume)
   size: 156315mb #s(320133744) s(39304-320173047)
   chs:  (1023/255/0)-(1023/255/0)d (0/0/0)-(0/0/0)r
   hex:  00 FF C0 FF 8E FF C0 FF 88 99 00 00 70 DA 14 13

Which looks right, but:

# losetup -o $((512*39304)) /dev/loop3 /dev/hde
# vgscan

Yields no new volume groups.  Neither does the above with 512*39303 or
512*39305 so it doesn't seem to be an off-by-one.  Nor does any of
512*(39303-8), 512*(39304-8), 512*(39305-8) to account for the fact that
loop2 (which the gpart was done on originally) is 8 blocks into /dev/hde
already.

Am I missing something?

Thanx,
b.

-- 
My other computer is your Microsoft Windows server.

Brian J. Murrell

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Re: [linux-lvm] recover a pv from a disk with no partition table

[David Brown]

On Mon, May 01, 2006 at 07:47:36PM -0400, Brian J. Murrell wrote:
> I need to recover a PV from a disk with no partition table.  I have a
> disk which gives EIO errors trying to read from the first few blocks of,
> so, no partition table.
> 
> Is there any way with LVM2, given that I have
> the /etc/lvm/backup/<vgname> file that I can determine the offset into
> the disk where the partition starts, so that I could do:

The backup files don't list physical offsets.  You'll have to figure that
out yourself.  But, you can see the id's for the partitions, and the lvm
header is at offset 0x200 into the partition.

Dave

Re: [linux-lvm] recover a pv from a disk with no partition table

[Brian J. Murrell]

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On Mon, 2006-05-01 at 16:55 -0700, David Brown wrote:
> On Mon, May 01, 2006 at 07:47:36PM -0400, Brian J. Murrell wrote:
> > I need to recover a PV from a disk with no partition table.  I have a
> > disk which gives EIO errors trying to read from the first few blocks of,
> > so, no partition table.
> > 
> > Is there any way with LVM2, given that I have
> > the /etc/lvm/backup/<vgname> file that I can determine the offset into
> > the disk where the partition starts, so that I could do:
> 
> The backup files don't list physical offsets.

Indeed.  That's why I used gpart, without success.

> But, you can see the id's for the partitions, and the lvm
> header is at offset 0x200 into the partition.

Yeah, I did notice that on another pv on the system:

# hexdump -C /dev/sda2 | less
00000000  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
00000200  4c 41 42 45 4c 4f 4e 45  01 00 00 00 00 00 00 00  |LABELONE........|
00000210  ad 24 02 c2 20 00 00 00  4c 56 4d 32 20 30 30 31  |­$. ...LVM2 001|
00000220  68 76 37 6b 4e 49 32 4e  72 5a 33 52 73 39 38 5a  |hv7kNI2NrZ3Rs98Z|
00000230  42 65 73 33 53 76 4a 4c  75 68 61 4f 62 6a 53 36  |Bes3SvJLuhaObjS6|

Unfortunately, I am unable to find any "LABELONE" on the PV I am trying
to recover.  I'm getting a bad feeling that that means that the LVM
(device mapper) mappings are gone.  :-(

b.

-- 
My other computer is your Microsoft Windows server.

Brian J. Murrell

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