mdadm superblock hiding/shadowing partition
- by Kjell Andreassen
Short version: Is it safe to do
mdadm --zero-superblock /dev/sdd
on a disk with a partition (dev/sdd1), filesystem and data?
Will the partition be mountable and the data still there?
Longer version:
I used to have a raid6 array but decided to dismantle it. The disks from the array are now used as non-raid disks.
The superblocks were cleared:
sudo mdadm --zero-superblock /dev/sdd
The disks were repartitioned with fdisk and filesystems created with mfks.ext4.
All disks where mounted and everything worked fine.
Today, a couple of weeks later, one of the disks is failing to be recognized when trying to mount it, or rather the single partition on it.
sudo mount /dev/sdd1 /mnt/tmp
mount: special device /dev/sdd1 does not exist
fdisk claims there to be a partition on it:
sudo fdisk -l /dev/sdd
Disk /dev/sdd: 2000.4 GB, 2000398934016 bytes
255 heads, 63 sectors/track, 243201 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: 0xb06f6341
Device Boot Start End Blocks Id System
/dev/sdd1 1 243201 1953512001 83 Linux
Of course mount is right, the device /dev/sdd1 is not there, I'm guessing udev did not create it because of the mdadm data still on it:
sudo mdadm --examine /dev/sdd
/dev/sdd:
Magic : a92b4efc
Version : 1.2
Feature Map : 0x0
Array UUID : b164e513:c0584be1:3cc53326:48691084
Name : pringle:0 (local to host pringle)
Creation Time : Sat Jun 16 21:37:14 2012
Raid Level : raid6
Raid Devices : 6
Avail Dev Size : 3907027120 (1863.02 GiB 2000.40 GB)
Array Size : 15628107776 (7452.06 GiB 8001.59 GB)
Used Dev Size : 3907026944 (1863.02 GiB 2000.40 GB)
Data Offset : 2048 sectors
Super Offset : 8 sectors
State : clean
Device UUID : 3ccaeb5b:843531e4:87bf1224:382c16e2
Update Time : Sun Aug 12 22:20:39 2012
Checksum : 4c329db0 - correct
Events : 1238535
Layout : left-symmetric
Chunk Size : 512K
Device Role : Active device 3
Array State : AA.AAA ('A' == active, '.' == missing)
My mdadm --zero-superblock apparently didn't work. Can I safely try it again without losing data? If not, are there any suggestion on what do to?
Not starting mdadm at all on boot might be a (somewhat unsatisfactory) solution.
