Recovering Data from Remaining RAID 1 Disk

nko · 05-02-2009, 09:48 PM

Hello,

A while back, I had a PC with two hard drives configured in a RAID 1 array with mdadm. I stored a lot of data there, though nothing too important. The OS installation went south at one point and I installed something over it. I left the RAID array untouched but notably didn't backup my mdadm.conf.

A year or so later, I now have a new PC with a new OS installation and only one of the disks remaining :-) I would like to mount the remaining disk, pull the data off and start using the disk for other purposes. Everything I'm reading tells me that I'm fine as long as I either have or recreate mdadm.conf AND as long as I go buy another disk to replace the missing one.

I've also read allusions to the ability to tell mdadm to mount an array with "missing disks" but haven't seen any instructions on how to do so. I've tried just popping the disk in and mounting it like a regular disk but mount refuses to do so, citing that it is of type "Linux RAID" or something thereof.

Is there a way to mount this disk and get the data?

Quakeboy02 · 05-02-2009, 10:28 PM

I think that you should be able to just mount it as a normal, non-raid, disk. Have you tried that? Use read only, of course.

nko · 05-02-2009, 10:32 PM

Yeah, doesn't let me. It isn't EXT3, it's "Linux RAID". That was my first guess :-)

tommylovell · 05-02-2009, 10:37 PM

Do a 'cat /proc/mdstat'. Your system may have recognized the drive when you booted up.
'fdisk -l' might show it as well.

If it is there, you can mount the "md" device shown in the cat (assuming you originally put your filesystem directly on it).
Example: 'mount /dev/md0 /<desiredmountpoint>'

If you put an LVM PV on top of it, it is only slightly more work (unless you have a VG name conflist).
You would 'pvscan', 'vgscan', 'vgchange -ay', 'lvs', then mount the LV.

If this isn't the case, let me know.

nko · 05-02-2009, 10:56 PM

Thanks for the pointers! It is definitely not LVM- just an FS. I wasn't able to mount. Here's my console output:

Code:

root@localhost:/# cat /proc/mdstat
Personalities : 
md0 : inactive sdc1[1](S)
      78148096 blocks
       
unused devices: <none>
root@localhost:/# fdisk -l

Disk /dev/sda: 4034 MB, 4034838528 bytes
255 heads, 63 sectors/track, 490 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Disk identifier: 0xb698b698

   Device Boot      Start         End      Blocks   Id  System
/dev/sda1   *           1         462     3710983+  83  Linux
/dev/sda2             463         490      224910    5  Extended
/dev/sda5             463         490      224878+  82  Linux swap / Solaris

Disk /dev/sdc: 80.0 GB, 80026361856 bytes
255 heads, 63 sectors/track, 9729 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Disk identifier: 0x415a13b8

   Device Boot      Start         End      Blocks   Id  System
/dev/sdc1               1        9729    78148161   83  Linux
root@localhost:/# mount /dev/md0 /mnt/md0
mount: you must specify the filesystem type
root@localhost:/# mount /dev/md0 /mnt/md0 -t autofs
mount: wrong fs type, bad option, bad superblock on /dev/md0,
       missing codepage or helper program, or other error
       (could this be the IDE device where you in fact use
       ide-scsi so that sr0 or sda or so is needed?)
       In some cases useful info is found in syslog - try
       dmesg | tail  or so

root@localhost:/# mount /dev/md0 /mnt/md0 -t ext3
mount: wrong fs type, bad option, bad superblock on /dev/md0,
       missing codepage or helper program, or other error
       (could this be the IDE device where you in fact use
       ide-scsi so that sr0 or sda or so is needed?)
       In some cases useful info is found in syslog - try
       dmesg | tail  or so

root@localhost:/#

Quakeboy02 · 05-02-2009, 11:51 PM

You should be trying to mount /dev/sdc1, not /dev/md0.

nko · 05-02-2009, 11:57 PM

Code:

root@localhost:/# mount /dev/sdc1 /mnt/md0
mount: unknown filesystem type 'linux_raid_member'
root@localhost:/# mount /dev/sdc1 /mnt/md0 -t ext3
mount: /dev/sdc1 already mounted or /mnt/md0 busy
root@localhost:/#

tommylovell · 05-02-2009, 11:58 PM

Quote:

Originally Posted by nko

Thanks for the pointers! It is definitely not LVM- just an FS. I wasn't able to mount. Here's my console output:

Code:

root@localhost:/# cat /proc/mdstat
Personalities : 
md0 : inactive sdc1[1](S)
      78148096 blocks
       
unused devices: <none>

Try 'mdadm --assemble --scan' and see if that makes md0 active. If it becomes active, then your 'mount /dev/md0 /mnt/md0
' should work.

(--scan says to use /proc/mdstat and /etc/mdadm.conf to look for eligible devices to assemble into a RAID array, and since your /proc/mdstat has your device, you should be good.)

If the md0 device doesn't become active, try 'mdadm --assemble --scan --run'.

Finally, if that doesn't work, you'll need to create an /etc/mdadm.conf file.

Typically you 'mdadm --examine --scan > /etc/mdadm.conf'

The resulting file contains something like this:

ARRAY /dev/md0 level=raid1 num-devices=2 UUID=397c7395:9c3c48fa:68e69357:c9f2169f

(The UUID will of course be different.)

You can edit it so it looks like this:

Code:

DEVICE partitions
MAILADDR root

ARRAY /dev/md0 level=raid1 num-devices=2 UUID=397c7395:9c3c48fa:68e69357:c9f2169f

or this:

Code:

DEVICE partitions
MAILADDR root

ARRAY /dev/md0 level=raid1 num-devices=2 devices=/dev/sdc1,missing

And then try 'mdadm --assemble --run'.

Let us know the results.

nko · 05-03-2009, 12:36 AM

Wow. Just had to assemble / scan and mount. That was faster, more concise tech support than any I've received from a lot of expensive vendors. Thanks, Tommy!