Methods and systems for cluster resource management in virtualized computing environments are described. VM spares are used to reserve (or help discover or otherwise obtain) a set of computing resources for a VM. While VM spares may be used for a variety of scenarios, particular uses of VM spares include using spares to ensure resource availability for requests to power on VMs as well as for discovering, obtaining, and defragmenting the resources and VMs on a cluster, e.g., in response to requests to reserve resources for a VM or to respond to a notification of a failure for a given VM.