While a new startup works to unofficially open source the VMware VMFS, Citrix has officially open sourced its implementation of the Microsoft VHD format.
Citrix and Microsoft adopt the same virtual hard drive format since September 2007, when they closed a deal to adopt VHD in all the upcoming products.
In over two years Citrix has developed an optimized implementation of the product and it’s now giving it back to the open source world by submitting its code to the Xen community for inclusion in the hypervisor code base under the BSD license.
If approved, Citrix partners and competitors that adopt Xen (like Virtual Iron, Oracle, Sun, etc.) will be able to use it side by side with QEMU Copy-On-Write (QCOW).