How to Create Virtual Block Devices

Question

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 ?

Answer 1

A better solution would be to set up a virtual machine and test in a VM, but this can be done without using a VM.

What you need to do is create a loopback device.

• Create a 100MB file: dd if=/dev/zero of=loopback_file1 bs=1M count=100
• Assign that file as a loopback device: losetup /dev/loop0 loopback_file1.

From then on the block device /dev/loop0 will appear like a drive, and the contents will go to the loopback_file1 file. You can then test your mdadm commands against that device.

Links:

• http://linux.die.net/man/8/losetup
• http://en.wikipedia.org/wiki/Loop_device
• http://www.walkernews.net/2007/07/01/create-linux-loopback-file-system-on-disk-file/