qemu-img create -b centos.5-8.x86.20120308.qcow2 -f qcow2 ../kvmguests/25000-centos5.8x86.qcow2
Formatting '../kvmguests/25000-centos5.8x86.qcow2', fmt=qcow2, backing_file=centos.5-8.x86.20120308.qcow2, size=10485760 kB

-b the source/base image

-f format is qcow2 and the location of the destination image

What is so special about this?  It's even quicker than creating a template with OpenVZ but this is an actual OS.

It saves time and disk space because it only writes the changes to the file of your image since the base OS/image is being used as a backing.  This means you could quickly and in an automated fashion create several servers for testing purposes for your applications or stress testing etc..

But there is one caveat and it's that you can't afford to lose the base image, it can't move or be modified/harmed otherwise the resulting images creating from it will stop working or have issues.  So it does introduce an unwanted and single point of failure, this is fine for testing but probably not wise to do for mission critical situations.

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