BOSH openstack stemcells are QCOW2 format by default. If you need RAW stemcell version. Use the following script.

#!/bin/bash

if [[ $# -eq 0 ]] ; then  
    echo 'Usage: raw_convert.sh <tgz stemcell>'
    exit 0
fi

original_dir=$(pwd)  
tmpdir=$(mktemp -d -t stemcell)  
cp $1 $tmpdir/  
pushd $tmpdir  
tar zxvf $1  
rm $1  
tar zxvf image  
qemu-img convert -O raw root.img root.raw  
mv root.raw root.img  
rm image  
tar czvf image root.img  
rm root.img  
sha1=$(openssl sha1 image| awk '{print $NF}')  
sed -i.bak "s/^sha1: .*$/sha1: $sha1/" stemcell.MF  
sed -i.bak "s/^  disk_format: .*$/  disk_format: raw/" stemcell.MF  
rm stemcell.MF.bak  
tar czvf $original_dir/bosh-stemcell-raw.tgz *  
popd  

Usage: ./raw_convert.sh bosh-stemcell-2605-openstack-kvm-ubuntu-lucid.tgz

To use RAW stemcell in microbosh/bosh boot from volume must be set.

Example:

cloud:  
  plugin: openstack
  properties:
    openstack:
      auth_url: http://10.1.0.1:5000/v2.0
      username: *****
      api_key: *****
      tenant: *****
      default_security_groups: ["ssh", "bosh"]
      default_key_name: microbosh
      private_key: /home/fedora/.ssh/microbosh
      boot_from_volume: true