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After a restart which stalled, I did a hard reboot and now mdadm can't see the RAID 6 array anymore, LVM can't find the volume which was on that raid array. I have looked into /etc/mdadm/mdadm.conf, it doesn't contain my /dev/md2 (the RAID 6 array).

How can I restore the RAID 6 array and then the LVM volume without losing any data?

OK, so I did an mdadm --assemble --scan and it re-assembled /dev/md2, but it missed one of the drives. The missing drive seems to have been added to a new, inactive array:

md_d2 : inactive sdb1[3](S)
1953511936 blocks

Going to remove it now and add manually to /dev/md2.

Still not sure about the missing LVM group & volume...

OK, after re-assembling the array, I got the LVM back:

root@freetunes:~# pvdisplay 
  --- Physical volume ---
  PV Name               /dev/md2
  VG Name               lvm-raid
  PV Size               5.46 TiB / not usable 43.50 MiB
  Allocatable           yes (but full)
  PE Size               256.00 MiB
  Total PE              22356
  Free PE               0
  Allocated PE          22356
  PV UUID               sJxydW-lMTJ-fsQn-Bln1-z6Cj-Dt6F-OPxMtr

root@freetunes:~# lvdisplay 
  --- Logical volume ---
  LV Name                /dev/lvm-raid/lvm0
  VG Name                lvm-raid
  LV UUID                D7Wj1A-wiJk-NxlM-IDdH-0uM4-aF1r-Fbpjjr
  LV Write Access        read/write
  LV Status              available
  # open                 0
  LV Size                5.46 TiB
  Current LE             22356
  Segments               1
  Allocation             inherit
  Read ahead sectors     auto
  - currently set to     1536
  Block device           251:0
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Are you relying on the kernel to auto-assemble the array for you? If so, double check that the volumes on ALL of the drives are set to the proper type. That's 0xfd for msdos partition tables or the raid flag for gpt tables. Here are example of each partition type properly setup for auto-assembly.

# parted /dev/sda
GNU Parted 1.8.1
Using /dev/sda
Welcome to GNU Parted! Type 'help' to view a list of commands.
(parted) print                                                            

Model: ATA INTEL SSDSA2M080 (scsi)
Disk /dev/sda: 80.0GB
Sector size (logical/physical): 512B/512B
Partition Table: msdos

Number  Start   End     Size    Type     File system  Flags     
 1      4194kB  273MB   268MB   primary  ext3         boot, raid
 2      273MB   80.0GB  79.8GB  primary               raid     

# parted /dev/sda
GNU Parted 2.4
Using /dev/sda
Welcome to GNU Parted! Type 'help' to view a list of commands.
(parted) print                                                            
Model: ATA WDC WD20EADS-32R (scsi)
Disk /dev/sda: 2000GB
Sector size (logical/physical): 512B/512B
Partition Table: gpt

Number  Start   End     Size    File system     Name  Flags
 1      262kB   1074MB  1074MB  ext3            boot  raid
 2      1074MB  35.4GB  34.4GB  linux-swap(v1)  swap  raid
 3      35.4GB  104GB   68.7GB  ext4            root  raid
 4      104GB   2000GB  1896GB                  vg00  raid
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