Ext4 and Software RAID1 follies

I am building some small servers. I wanted to stick a FS on a RAID1 configuration, so I created my disks like so:

• use md to create RAID1 config, create /dev/md0
• use fdisk to create partition /dev/md0p1
• use mkfs.ext4 to create fs on /dev/md0p1

This worked at runtime perfectly fine... could mount, create files, etc. But at boot time, I was getting errors complaining about superblocks and invalid ext2 filesystems.... odd... the disks won't automount and it drops me into a "Repair filesystem" shell (I'm running XenServer 6.2, similar to centOS 5 i believe) . I was still able to mount /dev/md0p1 manually, however.

Anyway, running out of ideas, I delete the partition, and create the ext4fs directly on /dev/md0. And.... it works!

My question to /r/linuxadmin is.... why would the presence of the partition make a difference? I'm stumped!

EDIT: grammar