From: NeilBrown <neilb@cse.unsw.edu.au>

With this patch, md used two major numbers for arrays.

One Major is number 9 with name 'md' have unpartitioned md arrays, one per
minor number.

The other Major is allocated dynamically with name 'mdp' and had on array for
every 64 minors, allowing for upto 63 partitions.

The arrays under one major are completely separate from the arrays under the
other.

The preferred name for devices with the new major are of the form:

  /dev/md/d1p3  # partion 3 of device 1 - minor 67

When a paritioned md device is assembled, the partitions are not recognised
until after the whole-array device is opened again.  A future version of
mdadm will perform this open so that the need will be transparent.