Paul Cobbaut: Recovering lvm2 on degraded mdadm raid 1

root@debian6~# fdisk -l /dev/sdb

Disk /dev/sdb: 1000.2 GB, 1000204886016 bytes
255 heads, 63 sectors/track, 121601 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x00060f19

   Device Boot      Start         End      Blocks   Id  System
/dev/sdb1               1         254     2040254+  83  Linux
/dev/sdb2             255      121602   974722329   83  Linux 

root@debian6~# fsck -n /dev/sdb2
fsck from util-linux-ng 2.17.2
fsck: fsck.linux_raid_member: not found
fsck: Error 2 while executing fsck.linux_raid_member for /dev/sdb2

root@debian6~# mdadm --examine /dev/sdb2
/dev/sdb2:
          Magic : a92b4efc
        Version : 1.0
    Feature Map : 0x0
     Array UUID : e8221214:354aaaf1:e9e15d78:075bfc18
           Name : storage:1
  Creation Time : Thu May  6 20:00:16 2010
     Raid Level : raid1
   Raid Devices : 2

 Avail Dev Size : 1949444384 (929.57 GiB 998.12 GB)
     Array Size : 1949444384 (929.57 GiB 998.12 GB)
   Super Offset : 1949444640 sectors
          State : clean
    Device UUID : 0c265a25:cc3f2ddd:ae7a1511:f447c74e

    Update Time : Tue May  1 23:09:11 2012
       Checksum : 498b9bd4 - correct
         Events : 33039098


   Device Role : Active device 0
   Array State : A. ('A' == active, '.' == missing) 

root@debian6~# mdadm --assemble --scan
mdadm: /dev/md/storage:1 has been started with 1 drive (out of 2).
mdadm: /dev/md/0_0 has been started with 1 drive (out of 2).

root@debian6~# cat /proc/mdstat 
Personalities : [linear] [multipath] [raid0] [raid1] [raid6] [raid5] [raid4] [raid10] 
md126 : active (auto-read-only) raid1 sdb1[0]
      2040128 blocks [2/1] [U_]

md127 : active (auto-read-only) raid1 sdb2[0]
      974722192 blocks super 1.0 [2/1] [U_]

unused devices:  

root@debian6~# fdisk -l
....
Disk /dev/md127: 998.1 GB, 998115524608 bytes
2 heads, 4 sectors/track, 243680548 cylinders
Units = cylinders of 8 * 512 = 4096 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x00000000

Disk /dev/md127 doesn't contain a valid partition table 

root@debian6~# mount /dev/md127 /mnt
mount: unknown filesystem type 'LVM2_member'

root@debian6~# lvs
  No volume groups found
root@debian6~# pvs

root@debian6~# vgscan
  Reading all physical volumes.  This may take a while...
  Found volume group "md1_vg" using metadata type lvm2

root@debian6~# vgs
  VG     #PV #LV #SN Attr   VSize   VFree
  md1_vg   1   1   0 wz--n- 929.57g    0 
root@debian6~# pvs
  PV         VG     Fmt  Attr PSize   PFree
  /dev/md127 md1_vg lvm2 a-   929.57g    0 
root@debian6~# lvs
  LV      VG     Attr   LSize   Origin Snap%  Move Log Copy%  Convert
  md1vol1 md1_vg -wi--- 929.57g 

root@debian6~# lvdisplay 
  --- Logical volume ---
  LV Name                /dev/md1_vg/md1vol1
  VG Name                md1_vg
  LV UUID                qSpslc-nO70-8jjX-5lBl-ge20-fFdR-dSEnWk
  LV Write Access        read/write
  LV Status              NOT available
  LV Size                929.57 GiB
  Current LE             475938
  Segments               1
  Allocation             inherit
  Read ahead sectors     auto 

root@debian6~# vgchange -a y
  1 logical volume(s) in volume group "md1_vg" now active

root@debian6~# mount /dev/md1_vg/md1vol1 /mnt

root@debian6~# mount | grep mnt
/dev/mapper/md1_vg-md1vol1 on /mnt type xfs (rw)