# Setting up a new RAID1 partition


The following might be helpful to others too. I was trying to setup a new
raid1 device from two partitions `/dev/sda4` and `/dev/sdb4`. I wanted to
do this the "right way" and use UUID everywhere, i.e. in
`/etc/mdadm/mdadm.conf` and in `/etc/fstab`. 

I hit a few snags along the way.

# create the array

    # mdadm --create --verbose /dev/md6 --level=1 --raid-devices=2 \
    /dev/sda4 /dev/sdb4

Get the uuid `mdadm` uses:

    # mdadm --detail  /dev/md6 | grep UUID
      UUID : dc9aba5e:ed1a70d4:770765d8:b0f56d86 (local to host elektron2)

Check. Add that to `/etc/mdadm/mdadm.conf`:

    # grep md6 < /etc/mdadm/mdadm.conf
    ARRAY /dev/md6 level=raid1 num-devices=2 UUID=dc9aba5e:ed1a70d4:770765d8:b0f56d86

Okay, that should take care of the array. Now I wanted to get it into
the `fstab`

# blkid

The magic command here is `blkid`. Let's run it:

    # blkid | grep md6

Huh, nothing.... hmmm. Okay maybe because there is no filesystem on it,
it won't show up?

    # mke2fs -j /dev/md0
    # blkid | grep md6
    /dev/md6: UUID="d4776336-0d4c-42e6-9b7f-a668522f3f40" SEC_TYPE="ext2" TYPE="ext3" 

Great, that did the trick. The UUID you see here can be put in `fstab`.
It is a completely different one than `mdadm` uses.

I've added the following to the `fstab`

    # /dev/md6
    UUID=d4776336-0d4c-42e6-9b7f-a668522f3f40 /vol ext3 relatime  0  2

Ok, lets mount it:

    # mount /vol
    mount: special device /dev/disk/by-uuid/d4776336-0d4c-42e6-9b7f-a668522f3f40 does not exist

WTF? So `blkid` is spitting out a UUID and `udev` hasn't updated the
links in `/dev`.... okay. Some 
[helpfull bug reports](http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=489230)
, [here](https://bugzilla.redhat.com/show_bug.cgi?id=465691) 
and
[here](https://bugzilla.redhat.com/show_bug.cgi?id=438604).

The one from Debian had the missing command:

    # echo add > /sys/block/md6/uevent

And low and behold, the symlink shows up in `/dev/disk/by-uuid/`

    # mount /vol
    # cd /vol
    # ls
    lost+found

Done.