How to Create Virtual Block Devices

I want to create multiple virtual block devices like /dev/sda and then use them to create an md device for test purpose. I have mounted my root filesystem on /dev/sda1 and don't have extra space to create new physical partitions. Therefore, I am looking for a way to temporary allocate small portion of my free space to these virtual block devices.

Can I use dmsetup or other utilities to create these virtual block devices ?

You can create a loopback device to a file and do it that way

# create a 100M file in /opt
dd if=/dev/zero of=/opt/dev0-backstore bs=1M count=100

# create the loopback block device 
# where 7 is the major number of loop device driver, grep loop /proc/devices
mknod /dev/fake-dev0 b 7 200 

losetup /dev/fake-dev0  /opt/dev0-backstore

Make a little script to automate this and done.