Ubuntu – Un-raiding a two-disk fakeraid array


I have a system with two disks in a bios-configured RAID-1 array, currently dual-booting Win7 and Ubuntu 10.10. I opted for fakeraid, rather than softraid, so that the raided disk could be seen from both Win7 (gaming) and Ubuntu (everything else!). For various reasons, I've decided to back out from running this system with a raided disk, so I would like two separate disks: one for the two OS versions, and one for data.

I removed the RAID setting in BIOS, and rebooted. Both OSes boot OK, but I can't tell what's happening with the disk config. When I list the mount table, I still the /dev/mapper entries that were familiar from fakeRAID:

$ sudo mount | grep /dev
/dev/mapper/pdc_beidbcaig5 on / type ext4 (rw,errors=remount-ro,commit=0)
none on /dev type devtmpfs (rw,mode=0755)
none on /dev/pts type devpts (rw,noexec,nosuid,gid=5,mode=0620)
none on /dev/shm type tmpfs (rw,nosuid,nodev)
/dev/dm-1 on /mnt/windows type fuseblk (rw,nosuid,nodev,allow_other,blksize=4096)
/dev/mapper/pdc_beidbcaig9 on /home type ext4 (rw,user_xattr,commit=0)
/dev/mapper/pdc_beidbcaig6 on /var type ext4 (rw,commit=0)
/dev/mapper/pdc_beidbcaig7 on /boot type ext3 (rw,commit=0)

If I start gparted, I can see the partitions of the two mirrored disks, listed separately for /dev/sda and /dev/sdb. However, when I ask for information on any partition displayed in gparted, I get a warning:

Warning: no such file or directory while trying to open /dev/sda5
Couldn't find valid system superblock

dumpe2fs 1.41.12 (17-May-2010)
dumpe2fs: No such file or directory while trying to open /dev/sda5

Unable to read the contents of this file system!

Now clearly the file system can be read, because otherwise it wouldn't boot. But equally clearly something is awry in the disk configuration. Sadly, I don't really know where to start.

I did read the man page for dmraid, and I thought that the -x option was what I needed. However:

$ sudo dmraid -x
About to delete RAID set pdc_beidbcaig
WARNING: The metadata stored on the raidset(s) will not be 
  accessible after deletion
Do you want to continue ? [y/n] :y
ERROR: Raid set deletion is not supported in "pdc" format

In summary, what steps should I take to ensure that my two previously mirrored RAID-1 disks become two separate independent drives, one of which I can reformat to store more data on?


Best Answer

OK, I found some helpful information in this thread. In particular, doing

sudo dmraid -rE

was helpful, though I had to manually patch /etc/fstab after that, reasonably enough. Still not sure that I've covered everything though, and I'd be happy to have an explanation of what dmraid does. Magic incantations to "make it all better" are OK, but I'd rather understand what I'm doing, at least in broad terms!

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