Best way to grow Linux software RAID 1 to RAID 10

Solution 1:

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

Solution 2:

Follow the same procedure as Mark Turner but when you create the raid array, mention 2 missing disks

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

And then proceed with other steps.

In short, create RAID10 with total 4 disks(out of which 2 are missing), resync, add other two disks after that.

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Solution 3:

Just finished going from LVM on two 2TB disk mdadm RAID 1 to LVM on a four disk RAID 10 (two original + two new disks).

As @aditsu noted the drive order is important when creating the array.

mdadm -v --create /dev/md1 --level=raid10 --raid-devices=4 /dev/sda missing /dev/sdb missing

Code above gives a usable array with two missing disks (add partition numbers if you aren't using whole disks). As soon as the third disk is added it will begin to sync. I added the fourth disk before the third finished syncing. It showed as a spare until the third disk finished then it started syncing.

Steps for my situation:

1. Make good backup.

2. Create a degraded 4 disk RAID 10 array with two missing disks (we will call the missing disks #2 and 4).

3. Tell wife not to change/add any files she cares about

4. Fail and remove one disk from the RAID 1 array (disk 4).

5. Move physical extents from the RAID 1 array to the RAID 10 array leaving disk 2 empty.

6. Kill the active RAID 1 array, add that now empty disk (disk 2) to the RAID 10 array, and wait for resync to complete.

7. Add the first disk removed from RAID 1 (disk 4) to the RAID 10 array.

8. Give wife go ahead.

At step 7 I think drive 1, 2, OR 4 can fail (during resync of disk 4) without killing the array. If drive 3 fails the data on the array is toast.