Linux mdadm software RAID 6 – does it support bit corruption recovery

With linux softraid you can make a RAID 10 array with only two disks.

Device names used below:

• md0 is the old array of type/level RAID1.
• md1 is the new array of type/level RAID10.
• sda1 and sdb2 are new, empty partitions (without data).
• sda2 and sdc1 are old partitions (with crucial data).

Replace names to fit your use case. Use e.g. lsblk to view your current layout.

0) Backup, Backup, Backup, Backup oh and BACKUP

1) Create the new array (4 devices: 2 existing, 2 missing):

mdadm -v --create /dev/md1 --level=raid10 --raid-devices=4 /dev/sda1 missing /dev/sdb2 missing

Note that in this example layout sda1 has a missing counterpart and sdb2 has another missing counterpart. Your data on md1 is not safe at this point (effectively it is RAID0 until you add missing members).

To view layout and other details of created array use:

mdadm -D /dev/md1

Note! You should save the layout of the array:

# View current mdadm config:
cat /etc/mdadm/mdadm.conf
# Add new layout (grep is to make sure you don't re-add md0):
mdadm --detail --scan | grep "/dev/md1" | tee -a /etc/mdadm/mdadm.conf
# Save config to initramfs (to be available after reboot)
update-initramfs -u

2) Format and mount. The /dev/md1 should be immediately usable, but need to be formatted and then mounted.

3) Copy files. Use e.g. rsync to copy data from old RAID 1 to the new RAID 10. (this is only an example command, read the man pages for rsync)

rsync -arHx / /where/ever/you/mounted/the/RAID10

4) Fail 1st part of the old RAID1 (md0), and add it to the new RAID10 (md1)

mdadm /dev/md0 --fail /dev/sda2 --remove /dev/sda2
mdadm /dev/md1 --add /dev/sda2

Note! This will wipe out data from sda2. The md0 should still be usable but only if the other raid member was fully operational.

Also note that this will begin syncing/recovery processes on md1. To check status use one of below commands:

# status of sync/recovery
cat /proc/mdstat
# details
mdadm -D /dev/md1

Wait until recovery is finished.

5) Install GRUB on the new Array (Assuming you're booting from it). Some Linux rescue/boot CD works best.

6) Boot on new array. IF IT WORKED CORRECTLY Destroy old array and add the remaining disk to the new array.

POINT OF NO RETURN

At this point you will destroy data on the last member of the old md0 array. Be absolutely sure everything is working.

mdadm --stop /dev/md0
mdadm /dev/md0 --remove /dev/sdc1
mdadm /dev/md1 --add /dev/sdc1

And again - wait until recovery on md1 is finished.

# status of sync/recovery
cat /proc/mdstat
# details
mdadm -D /dev/md1

7) Update mdadm config

Remember to update /etc/mdadm/mdadm.conf (remove md0).

And save config to initramfs (to be available after reboot)

update-initramfs -u

Looks like your drives are all being reported as [S]pares. You should check your logs (dmesg, /var/log/messages) to see if there's any indication why this happened.

Try running the following

sudo mdadm --examine --scan --config=/etc/mdadm/mdadm.conf

And see the output. If it outputs something like this:

ARRAY /dev/md0 level=raid5 metadata=1 num-devices=3 UUID=22f22c3599:613d5231:d407a655:bdeb84 name=backup:1

Then you can append it to the bottom of the mdadm.conf:

sudo mdadm --examine --scan --config=/etc/mdadm/mdadm.conf >> /etc/mdadm/mdadm.conf

Then try starting the array:

sudo mdadm -A /dev/md0

Good luck.