Ubuntu – How to Create a RAID1 Partition with mdadm?

linuxmdadmraidsoftware-raidUbuntu

I'm trying to set up a RAID1 partition on my Ubuntu 9.10 workstation.

On this dual-boot system, Ubuntu is running from a separate drive (/dev/sdc – an SSD that is quite small, which is why I need more disk space). Besides that, there are two traditional 500 GB hard drives, which have Windows 7 installed (I want to keep the Windows installation intact), and about half of the space unallocated. This space is where I want to set up a single, large RAID1 partition for Linux.

(This, to my understanding, would be software RAID, whereas the Windows partitions are on hardware RAID – I hope this isn't a problem… Edit: See Peter's comment. I guess this shouldn't be a problem since I see both drives separately on Linux.)

On both disks, /dev/sda and /dev/sdb, I created, using fdisk, identical new partitions of type "Linux raid autodetect" to fill up the unallocated space. 

   Device Boot      Start         End      Blocks   Id  System
/dev/sda1               1          10       80293+  de  Dell Utility
/dev/sda2   *          11         106      768000    7  HPFS/NTFS
Partition 2 does not end on cylinder boundary.
/dev/sda3             106       30787   246439936    7  HPFS/NTFS
/dev/sda4           30787       60801   241095200+  fd  Linux raid autodetect

So, I would like to create a RAID array from /dev/sda4 and /dev/sdb4 using mdadm. But I don't seem to get it working:

$ sudo mdadm -Cv /dev/md0 -l1 -n2 /dev/sd{a,b}4
mdadm: Cannot open /dev/sda4: Device or resource busy
mdadm: Cannot open /dev/sdb4: Device or resource busy
mdadm: create aborted

After booting the machine, the same command yields:

$ sudo mdadm -Cv /dev/md0 -l1 -n2 /dev/sda4 /dev/sdb4
mdadm: Cannot open /dev/sda4: No such file or directory
mdadm: Cannot open /dev/sdb4: No such file or directory

So now it seems that the devices are not automatically detected in boot… Using fdisk both sda and sdb still look correct though.

Edit: After another reboot the devices are back: 

$ ls /dev/sd*
/dev/sda   /dev/sda2  /dev/sda4  /dev/sdb1  /dev/sdb3  /dev/sdc   /dev/sdc2
/dev/sda1  /dev/sda3  /dev/sdb   /dev/sdb2  /dev/sdb4  /dev/sdc1  /dev/sdc5

But so is "Device or resource busy" when trying to create the RAID array. Quite strange. Any help would be appreciated!

Update: Could the device mapper have something to do with this? How do /dev/mapper and dmraid relate to all this mdadm stuff anyway? Both provide software RAID, but.. differently? Sorry for my ignorance here.

Under /dev/mapper/ there are some device files that, I think, somehow match the 3 Windows RAID partitions (sd{a,b}1 through sd{a,b}3). I don't know why there are four of these arrays though. 

$ ls /dev/mapper/
control               isw_dgjjcdcegc_ARRAY1  isw_dgjjcdcegc_ARRAY3
isw_dgjjcdcegc_ARRAY  isw_dgjjcdcegc_ARRAY2

Resolution: It was the mdadm Wikipedia article that pushed me in the right direction. I posted details on how I got everything working in this answer.

Best Answer

Finally some progress!

dmraid indeed was the culprit, as mdadm's Wikipedia entry suggested. I tried removing dmraid packages (and running update-initramfs though I'm not sure if that was relevant).

After that, and rebooting, the devices under /dev/mapper are gone (which is fine - I don't need to access the Windows NTFS partitions on Linux):

$ ls /dev/mapper/
control

And, most importantly, mdadm --create works!

$ sudo mdadm -Cv -l1 -n2  /dev/md0 /dev/sda4 /dev/sdb4
mdadm: size set to 241095104K
mdadm: array /dev/md0 started.

I checked /proc/mdstat and mdadm --detail /dev/md0 and both show that everything is fine with the newly created array. 

$ cat /proc/mdstat
[..] 
md0 : active raid1 sdb4[1] sda4[0]
      241095104 blocks [2/2] [UU]
      [==========>..........]  resync = 53.1% (128205632/241095104) 
        finish=251.2min speed=7488K/sec

Then I created a filesystem on the new partition:

$ sudo mkfs.ext4 /dev/md0

And finally just mounted the thing under /opt (& updated /etc/fstab). (I could of course have used LVM here too, but frankly in this case I didn't see any point in that, and I've already wasted enough time trying to get this working...)

So now the RAID partition is ready to use, and I've got plenty of disk space. :-)

$ df -h
Filesystem            Size  Used Avail Use% Mounted on
/dev/sdc5              70G   52G   15G  79% /
/dev/md0              227G  188M  215G   1% /opt

Update: there are still some issues with this RAID device of mine. Upon reboot, it fails to mount even though I have it in fstab, and sometimes (after reboot) it appears to be in an inactive state and cannot be mounted even manually. See the follow-up question I posted.

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