Data processing system using skeleton virtual volumes for improved system startup

A computer system includes a virtual machine (VM) host computer and a data storage system providing physical storage and mapping logic to store a virtual volume (vVol) for a VM. During a first operating session, first-session working data is stored on the vVol, the working data being session specific and not persisting across operating sessions. At the end of the first operating session, unmap operations are performed to deallocate underlying physical storage of the vVol, leaving the vVol as a skeleton vVol. At the beginning of a subsequent second operating session, and based on the existence of the vVol as the skeleton vVol, the VM host resumes use of the vVol for storing second-session working data of the VM during the second operating session. The retention of the vVol in skeleton form can improve system startup efficiency especially for a boot storm involving simultaneous startup of many VMs.