Debian Jessie RAID6 recovery mdadm

Question

I have a Debian Jessie box that lost the RAID volume that had the OS, so I booted Live USB to try to recover the attached SAS controlled external drive bay which has my data RAID6 and it sees 3 of the 4 like:

>: cat /proc/mdstat
Personalities :
md0 : inactive sda1[0](S) sdc1[2](S) sdb1[1](S)
  11718349824 blocks super 1.2

unused devices: <none>

and again:

>: mdadm -D /dev/md0
/dev/md0:
    Version : 1.2
 Raid Level : raid0
Total Devices : 3
Persistence : Superblock is persistent

      State : inactive

       Name : backup1:0
       UUID : a7946015:259ae101:1fed525f:5766e9d5
     Events : 381

Number   Major   Minor   RaidDevice

   -       8        1        -        /dev/sda1
   -       8       17        -        /dev/sdb1
   -       8       33        -        /dev/sdc1

So it thinks it's a weird RAID0? Is there a way to tell it to use this as a RAID6 again, but not delete data and not mark drives as spares? I'm thinking something like:

mdadm --stop /dev/md0
mdadm --create /dev/md0 --level=6 --raid-devices=4 --chunk=64 --name=backup1:0 /dev/sda1 /dev/sdb1 /dev/sdc1 /dev/sdd1 --assume-clean

even though /dev/sdd1 isn't being shown. It almost seems like I need to re-assemble with some flags to avoid rewriting data, or manually remove/add each disk from md0 (but you need multiple disks for RAID6 so how do you do that?) fdisk for /dev/sdd shows:

Disk /dev/sdd: 3.7 TiB, 3999999721472 bytes, 7812499456 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: gpt
Disk identifier: 27419FEB-5830-4C44-9DC9-00828D0F115A

Device     Start        End    Sectors  Size Type
/dev/sdd1   2048 7812497407 7812495360  3.7T Linux RAID

So there's a raid partition there like I expect, though when I examine it, it shows:

mdadm --examine /dev/sdd1
mdadm: No md superblock detected on /dev/sdd1.

unlike other drives:

mdadm --examine /dev/sdc1
/dev/sdc1:
      Magic : a92b4efc
    Version : 1.2
Feature Map : 0x1
 Array UUID : a7946015:259ae101:1fed525f:5766e9d5
       Name : backup1:0
Creation Time : Tue Jul 19 17:34:55 2016
 Raid Level : raid6
Raid Devices : 4

Avail Dev Size : 7812233216 (3725.16 GiB 3999.86 GB)
 Array Size : 7812233216 (7450.33 GiB 7999.73 GB)
Data Offset : 262144 sectors
Super Offset : 8 sectors
Unused Space : before=262056 sectors, after=0 sectors
      State : active
Device UUID : 4d1d775e:eef629d4:03f15e09:f1762443

Internal Bitmap : 8 sectors from superblock
Update Time : Tue Jul 19 18:17:36 2016
Bad Block Log : 512 entries available at offset 72 sectors
   Checksum : 8365777c - correct
     Events : 381

     Layout : left-symmetric
 Chunk Size : 512K

Device Role : Active device 2
Array State : AAAA ('A' == active, '.' == missing, 'R' == replacing)

I guess during the rebuild it could repair /dev/sdd1? Am I approaching this right? I don't want to overwrite data basically.

score 0 · Answer 1 · answered Mar 01 '18 at 22:11

Okay, I stopped the /dev/md0 and then created it as raid6 like:

mdadm --stop /dev/md0
mdadm: stopped /dev/md0
root@debian:/home/user >: cat /proc/mdstat
Personalities :
unused devices: <none>

root@debian:/home/user >: mdadm --create /dev/md0 --level=6 --raid-devices=4 --chunk=64 --name=backup1:0 /dev/sda1 /dev/sdb1 /dev/sdc1 /dev/sdd1 --assume-clean
mdadm: /dev/sda1 appears to be part of a raid array:
   level=raid6 devices=4 ctime=Tue Jul 19 17:34:55 2016
mdadm: /dev/sdb1 appears to be part of a raid array:
   level=raid6 devices=4 ctime=Tue Jul 19 17:34:55 2016
mdadm: /dev/sdc1 appears to be part of a raid array:
   level=raid6 devices=4 ctime=Tue Jul 19 17:34:55 2016
Continue creating array? y
mdadm: Defaulting to version 1.2 metadata
mdadm: array /dev/md0 started.

root@debian:/home/user >: cat /proc/mdstat
Personalities : [raid6] [raid5] [raid4]
md0 : active raid6 sdd1[3] sdc1[2] sdb1[1] sda1[0]
  7812233216 blocks super 1.2 level 6, 64k chunk, algorithm 2 [4/4] [UUUU]
  bitmap: 30/30 pages [120KB], 65536KB chunk

unused devices: <none>

Here's more detail on the re-assembled RAID:

mdadm -D /dev/md0
/dev/md0:
    Version : 1.2
Creation Time : Thu Mar  1 22:04:57 2018
 Raid Level : raid6
 Array Size : 7812233216 (7450.33 GiB 7999.73 GB)
Used Dev Size : 3906116608 (3725.16 GiB 3999.86 GB)
Raid Devices : 4
Total Devices : 4
Persistence : Superblock is persistent
Intent Bitmap : Internal

Update Time : Thu Mar  1 22:04:57 2018
      State : clean
Active Devices : 4
Working Devices : 4
Failed Devices : 0
Spare Devices : 0

     Layout : left-symmetric
 Chunk Size : 64K

       Name : backup1:0
       UUID : c3ef766b:2fe9581a:5a906461:d52ee71e
     Events : 0

Number   Major   Minor   RaidDevice State
   0       8        1        0      active sync   /dev/sda1
   1       8       17        1      active sync   /dev/sdb1
   2       8       33        2      active sync   /dev/sdc1
   3       8       49        3      active sync   /dev/sdd1

So I guess it worked! Now when I go to mount it, it says

root@debian:/home/user >: mount /dev/md0 raid6
mount: unknown filesystem type 'LVM2_member'

So now I have to figure out how to rebuild LVM stuff, but that's a separate issue. I hope this helps someone else working through something like this.

Debian Jessie RAID6 recovery mdadm

1 Answers1