There has been a fair amount of conversation and blog posts regarding an issue with the VMware Virtual Disk Development Kit (VDDK) for vSphere 5.1 that may cause backup and restore operations to hang or fail. This is no surprise considering successful backup and recovery of virtual machines is mission-critical to many organizations.
To provide a better understanding, let's first take a brief look at how the VDDK is utilized by many virtual machine backup and recovery solution vendors: These vendors are permitted to repackage VDDK binaries as part of their solution (provided they sign a redistribution agreement). As new versions of the VDDK are released, backup and recover vendors update their software to include and utilize the new VDDK binaries to protect virtual machines running on the latest versions of vSphere.
VMware naturally released the VDDK 5.1 binaries with vSphere 5.1 to ensure continuity in backup and recovery support via third-party solutions. Shortly after this release, some backup partners reported bugs with the APIs. From the moment these bugs were discovered, VMware has been working and continues to work with impacted partners to enable support for vSphere 5.1 platform by leveraging the VDDK 5.0 U1 APIs. This helps ensure that customers can continue to confidently backup and successfully restore data in their VMware environments. Fixes for the reported issues have been identified and will soon be addressed in an updated version of VDDK 5.1. In the meantime, it is recommended you contact your backup partner simply to verify their vSphere 5.1 support statement.
VMware KB article: http://kb.vmware.com/kb/2039931
More information on VDDK: http://www.vmware.com/support/developer/vddk/