1. A method of archiving virtual machines in a networked data storage system, comprising: using one or more computing devices comprising computer hardware: monitoring usage of a virtual machine (VM) executing on one or more client computing devices in a primary storage subsystem, the VM associated with a virtualization application, wherein said monitoring is based at least in part on usage of resources associated with the VM, the resources including one or more of: central processing unit (CPU), memory, storage, or network, and wherein said monitoring is performed according to a storage policy indicating a threshold resource usage value for archiving the VM; in response to determining that the usage of the VM is less than or equal to the threshold resource usage value, using one or more secondary storage controller computers comprising computer hardware, copying the VM from one or more primary storage devices residing in the primary storage subsystem to one or more secondary storage devices residing in a secondary storage subsystem to create a secondary copy of the VM; creating a VM placeholder for the VM on the one or more primary storage devices; and deleting the VM from the one or more primary storage devices, wherein, subsequent to the deletion of the VM, the VM placeholder is recognized as the VM by the virtualization application and the secondary copy of the VM is accessed using the VM placeholder, and wherein said monitoring usage of a VM, said copying the VM, said creating the VM placeholder, and said deleting the VM occur during a backup of data from the primary storage subsystem to the secondary storage subsystem.