Drbd block device as storage for kvm virtual machine

Well, seeing as though I pulled my hair out solving this, let me answer my question and save someone else the trouble of hair pulling :)

Solution:

After playing around, plenty of Googling and repartitioning etc... I came to a setup that works like a charm.

There is probably a quicker way to do this but I am not going to over complicate this answer

I did a standard install with the partitions like this (I have a 500g hard drive) :

/boot 100mb 
/swap 4gb 
/     40gb 

The balance of the disk space is to be left as unpartitioned space.

Then, I created a primary partition called /dev/sda4 by following these steps:

~: fdisk /dev/sda
~: (fdisk shell) p4   (for primary partition # 4)
~: (fdisk shell) t    (hit t and enter to edit the partition type)
~: (fdisk shell) 08e  (Linux LVM)

Reboot the server so that the new partitions can take effect.

Now create logical volumes by:

~: pvcreate /dev/sda4
~: vgcreate xenvg -s 4M /dev/sda4    # (xenvg is the name of my virtual group, you can rename it)
~: lvcreate -L400G -n xenroot xenvg  # (xenroot is going to be my drbd  resource and root partition for my DomU)
~: lvcreate -L4G  -n xenswap xenvg   # (xenswap is my swap file for my DomU)

Now that you have the correct partitioning you can go ahead and install DRBD with the following config file directives (drbd.conf)

Just displaying the 2 important directives here...

{
device  /dev/drbd0;
disk    /dev/xenvg/xenroot;
}

Your XEN VM config file needs to look like this (again, just the important one)

{
disk = [ "drbd:xenvm,xvda,w","phy:xenvg/xenswap,xvdb,w" ]
}

I hope this helps someone out there...

First, create a raw image of the required size. I'll assume 10G is enough. Using seek creates a sparse file, which saves space.

dd if=/dev/null of=example.img bs=1M seek=10240

Next, create a filesystem on it.

mkfs.ext4 -F example.img

(Note that you need the -F option for mkfs.ext4 to operate on a file as opposed to a disk partition)

Then, mount it.

mkdir /mnt/example
mount -t ext4 -o loop example.img /mnt/example

Now you can copy your files to /mnt/example. Once this is done, unmount it and you can use example.img as a drive in a virtual machine. If you want you can convert it from a raw image to another format like qcow2e using qemu-img, but this isn't required.